speedo reading a bit slow

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peter2b
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speedo reading a bit slow

Post by peter2b » Mon 13 Apr, 2015 18:32

Just taken my z3 out on a private road for my first ever spin, goes very nicely but i think the speedo might be out in 4th at 2k revs the speedo says 40mph is that correct?, still got a smile on my face
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colb
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Re: speedo reading a bit slow

Post by colb » Mon 13 Apr, 2015 19:46

You might try the instrument test function details how to access it here
http://www.zroadster.net/forum/viewtopi ... on#p136496

Access the vehicle speed test from the menu and go for a drive to monitor speedo and display.

You could also compare the speedo using a Satnav displaying the speed.

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Southernboy
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Re: speedo reading a bit slow

Post by Southernboy » Mon 13 Apr, 2015 20:02

......... drive faster.... it'll catch up... :lol:
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BladeRunner919
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Re: speedo reading a bit slow

Post by BladeRunner919 » Tue 14 Apr, 2015 09:41

I agree about using a sat-nav. That gives you a reading independent of anything to do with the car.

Mike Fishwick
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Re: speedo reading a bit slow

Post by Mike Fishwick » Tue 14 Apr, 2015 11:55

40 mph at 2000 rpm works out at 20 mph/1000 rpm, which gives 100 mph at 5000 rpm in fourth gear. This is the same as my 2.8, so sounds normal.
As you can see, the Z3 is quite low geared - probably to please the Americans, who desire the social kudos of having a five-speed manual box, but want to start off in second gear, and go almost everywhere in fifth, as if it were an auto box.

It's not necessarily a bad thing though, as under 'real world' conditions this gives the Z3 a lot of flexibility and enviable in-gear accelleration ability. For example, when compared to the 2.5 litre Porsche Boxster of the same period, the 2.8 has far better 'foot down' performance.

An Autocar road test of September 1997 recorded 50 to 90 mph times in top gear of 8.8 seconds for the Z3, and 12.3 for the Boxster -almost 4 seconds at 90 mph putting the Z3 almost 100 yards further down the road! Differences of the same proportion were found throughout all speed ranges in all three upper gears.

What this means is that in overtaking situations where the Z3 can simply be squirted in top gear, the Boxster driver would have to drop a gear - sometimes two gears - to keep up. This was why Porsche brought out the 3 litre Boxster S, but BMW then produced the 3 litre Z3 . . .

In fifth gear I get 25 mph/1000 rpm, which gives 50 mph at 2000 rpm etc. leading to 137 mph at 5500 rpm. All these speeds are approximate, as both the speedometer and tachometer have their own built-in errors. While the speedometer is digital from the final drive to the digital-to-analogue converter which feeds the speedometer, it is still subject to the usual errors of all such moving-coil instruments, in addition to that caused by changes in the circumference of the rear tyres due to wear, or the fitting of larger-size tyres for cosmetic purposes.

In terms of basic accuracy, athough the UK legal tolerance for a speedometer is +10% BMW state that their speedometers should be within +5%.
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peter2b
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Re: speedo reading a bit slow

Post by peter2b » Tue 14 Apr, 2015 19:42

Cheers for that info, when i droped down from 4th to 3rd floored the throttle the back end squated front lifted and pushed me back into the seat and swmbo sqeeled
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motco
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Re: speedo reading a bit slow

Post by motco » Wed 15 Apr, 2015 10:26

I have a 2.2 as well, and I am suspicious that my speedo is reading too high so I'll check the 20mph/1000rpm in 4th figure. My TomTom says about 8mph less than the dial at 70mph. My tyre pressures were significantly down, though, and that will contribute. I must make certain they're right when I do this test. I have been having instrument cluster problems since having a dead flat battery though - odometer reads EEP-1 and no tacho or fuel gauge until it's restarted after a run. :shake:

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Re: speedo reading a bit slow

Post by Mike Fishwick » Wed 15 Apr, 2015 13:00

It is worth doing the GPS check a few times, on different roads at steady indicated speeds, as GPS signal accuracy can vary a bit, being dependant upon what is going on in the upper atmosphere, and the number of satellites currently visible. The 0.5 mph accuracy often quoted by manufacturers is a 'Best case' figure, which can be degraded by many factors at ground level.
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motco
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Re: speedo reading a bit slow

Post by motco » Wed 15 Apr, 2015 15:57

Mike Fishwick wrote:It is worth doing the GPS check a few times, on different roads at steady indicated speeds, as GPS signal accuracy can vary a bit, being dependant upon what is going on in the upper atmosphere, and the number of satellites currently visible. The 0.5 mph accuracy often quoted by manufacturers is a 'Best case' figure, which can be degraded by many factors at ground level.
I'm sure you're right Mike. Apart from those factors you mention, there's the need for a straight and level (neither ascending nor descending) road and steady speed for several sampling cycles (whatever they are), and the possibility of multi-path signals - i.e. reflection and deflection by local surfaces above antenna level.

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Re: speedo reading a bit slow

Post by Mike Fishwick » Wed 15 Apr, 2015 16:13

The sampling rate for GPS is several times a second, so the old bogey of the system thinking a straight line is being pursued when you are driving around a twisty road is not a real problem.

In the old days - the 'seventies - the original Satnav system, Transit, could give very interesting average speeds if you missed a satellite acquisition for any reason - they were in polar orbits, and the interval between 'seeing' a satellite reduced markedly as you approached northern latitudes, and increased in equatorial areas. The receiver/computer/ancilliaries were about as big as a single wardrobe, too!
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Re: speedo reading a bit slow

Post by peter2b » Wed 15 Apr, 2015 19:00

I will try my sat nav, untill last year i used my sat nav on my kit car as i could not find why my speedo would not work at all i used it for 2 trips to Le Mans and 3 MOTs
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Re: speedo reading a bit slow

Post by bertiejaffa » Thu 16 Apr, 2015 00:49

checked my 2.0l yesterday.... 4th gear.... 40mph... 2,000 rpm - its fine
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peter2b
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Re: speedo reading a bit slow

Post by peter2b » Thu 16 Apr, 2015 09:37

thanks that's what my rev/speedo reads, just taken her to the garage for her MOT on they way there, Robert T was correct my engine light went off so that's 1thing less to worry about
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motco
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Re: speedo reading a bit slow

Post by motco » Thu 16 Apr, 2015 12:13

I've now checked my 2.2 and it's c.42 @ 2k rpm, and 62 @ 3k rpm both in 4th gear so not as far out as I feared. I suppose that unless you are using OEM tyres then there is bound to be a discrepancy.

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Re: speedo reading a bit slow

Post by Mike Fishwick » Thu 16 Apr, 2015 12:35

No - the speedometer system loses interest in anything past the final drive, so it does not care about the size or type of tyres you are using! It just uses the final drive gear ratio and the frequency of the pulses from the speed sensor. I have not calculated the exact mph per 1000 rpm figure for each gear, the 20 mph and 25 mph figures being an approximation - the true figure may even agree with your speedometer.

It would be interesting to see what the speedometer error is when compared to a GPS speed.

People who are used to using a satnav tell me that Garmin have stated that the vehicle speedometer is probably more accurate because the signals from the satelite may not come direct to the vehicle, and this will delay and indication on the device. Users report a discrepancy of 2-4 mph at speeds between 20mph and 70mph, and sometimes less on very twisty roads or long steep hills, when the point to point distance of the GPS is shorter than the actual distance travelled.

I'm still happy using my eyes, memory, maps, and daily idiot cards, so I have a good grasp of the layout in whatever area I am driving - so many GPS addicts can get to any destination, but when they reach it they do not know where they are, or how they got there!
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Re: speedo reading a bit slow

Post by peter2b » Thu 16 Apr, 2015 14:33

MOT failed, both front indicates wrong colour and near side head light loose, not too bad, I can't see why the indicaters are wrong colour I used clear bulbs behind coulourd lenses

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motco
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Re: speedo reading a bit slow

Post by motco » Thu 16 Apr, 2015 15:42

I agree about satnav and loss of sense or geography, Mike, but a I cannot agree with you on tyre sizes. If by some means you increased the rolling circumference of your wheels by 50%, say, then the distance travelled per turn of the wheels, and by direct connection turn of the tailshaft from the gearbox, would increase pro rata. Therefore the speedometer would read low. To take it to the extreme if the wheels were twice the rolling circumference the speedo would read half the road speed.

My Westfield has a VDO speedo that reads 66% of real road speed for some reason, but because I'm a tightwad and wouldn't pay Speedy Cables to re-gear it, I bought a bike speedometer from Halfords for a tenner. I had to program it for the wheel circumference to make it accurate but it does the job perfectly if I realise the sampling rate is slow. A magnet on the o/s driveshaft Lobro joint activates a reed switch in a sensor which ticks up a counter against a clock chip. All for £10 - marvellous! Checked against other cars and satnav it is pretty accurate.

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Re: speedo reading a bit slow

Post by Mike Fishwick » Thu 16 Apr, 2015 17:11

Yes - that is what I said! The speedo will always read the same, irrespective of the tyre pressures, and therefore size of rear wheel in use. The relationship between the rpm of the prop shaft and rear wheel driveshafts - and therefore the speedometer reading - is not affected by the circumference of the rear tyres. If you fit a larger rear tyre the speedo will still read at 25 mph/1000 rpm in top gear. As I stated:

'The speedometer system loses interest in anything past the final drive, so it does not care about the size or type of tyres you are using! It just uses the final drive gear ratio and the frequency of the pulses from the speed sensor.'

You could fit a larger rear tyre/wheel giving, say, 50 mph/1000 rpm, but the speedometer reading would still be appropriate to the usual 25 mph/1000 rpm ratio - other words it will read low. If you fit a rear tyre with a smaller ciircumference the speedometer will read fast, for each turn of the rear wheels will present a smaller distance of tyre tread to the road - but the speedometer will still work at the usual rate.
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motco
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Re: speedo reading a bit slow

Post by motco » Thu 16 Apr, 2015 18:28

Ah Mike, we were at cross purposes because I was (albeit none too clearly) referring back to the discrepancy in real speed vs indicated speed rather than the tacho vs speedo readings. So we are in agreement and harmony is restored! :)

As a point of interest I have now checked 80mph :twisted: and found that 4k rpm reads c.78 or 79. Odd because at 2k and 3k it read a little over. When an opportunity arises I'll use my TomTom to check against my X-Type speedo and then cross reference it to the Z.

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Robert T
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Re: speedo reading a bit slow

Post by Robert T » Thu 16 Apr, 2015 19:04

Put another way, the speedometer measures the rate of rotation of the propshaft/driveshaft, whereas the actual speed of the vehicle is a function of how far each rotation of the wheel takes it. This means that the speedo is only accurate when the vehicle is fitted with the manufacturers recommended size of tyres (or at least ones with the same rolling radius).

BMW speedos seem to be setup to over-read by about 5mph at 70 mph. So if your speedo reads 70mph, then you are more likely to be travelling at around 65mph. My satnav confirms this on my car.

Cheers R.
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Re: speedo reading a bit slow

Post by peter2b » Thu 16 Apr, 2015 20:36

Just checked the failed note for the MOT it was the rear indicator lights both bulbs had lost thier coating so showed too much white ,i also forgot to put all the screws back that held the near side head light in so it was a bit loose so failed the head light test ,never mind tacking her back on saturday for a retest, speedo IS working ok now it was me who thought it was wrong just seemed to going slower that i thought it should
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Re: speedo reading a bit slow

Post by Brian4 » Fri 17 Apr, 2015 19:33

' speedo IS working ok now it was me who thought it was wrong just seemed to going slower that i thought it should ' I guess from that statement that you daily drive is a diesel ours is 1750 at 80 in 8th
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Re: speedo reading a bit slow

Post by sirius-c » Fri 17 Apr, 2015 21:09

Mike Fishwick wrote: It would be interesting to see what the speedometer error is when compared to a GPS speed.

I currently use an android app called DigiHUD Pro by James Moss.
There is an option under the settings menu called "Speed Offset", I have mine set to +2%.
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Re: speedo reading a bit slow

Post by Southernboy » Sat 18 Apr, 2015 08:35

My Garmin provides a repetitive "ping" when I exceed the speed limit... I have noticed that the speedo is a fraction optimistic compared to the Garmin. ie. the speedo will show 85kph when the Garmin tells me I'm just over 80kph.
We have 3 predominant "general" speed restrictions here - 60kph for suburban streets, 80kph for dual carriageways, and 120kph for motorways. There is an unofficial "DILLIGAF" speed though.... :D
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Re: speedo reading a bit slow

Post by peter2b » Sat 18 Apr, 2015 13:04

2nd trip out for mot's getting a bit more use to her comming back from the garage just over took a land rover tickling along at 30 next looked at the speedo i hit 70 before i new it, just booked her in for a vic check
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motco
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Re: speedo reading a bit slow

Post by motco » Thu 20 Aug, 2015 09:14

Thread revival! I was in a position to check my speedo against two GPS devices yesterday and found that it reads about 10mph high at 70mph and about 6mph high at 50mph. As I didn't have any suspicions about it before my EEP_1 episode (still extant when cold), and having had the car since 2009 I know how 30mph feels, I can only assume the two phenomena are interrelated. Unless I am prepared to pay BMW prices for a replacement cluster (I am not) Ebay has to be the way forward. :(

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Re: speedo reading a bit slow

Post by Southernboy » Thu 20 Aug, 2015 09:46

Before you go off to get another cluster, you must realise there is no "perfect" reading as far as cluster dials go. Also,the fault could originate from the speed sensor. It is fitted into the diff. This relays the axle speed to the DME, which translates the message to the cluster where it is reflected on the speedo dial.
Second, the speedo needle indicator is fitted to a shaft which is sprung. The spring returns the needle to zero when you stop. That spring coils up tighter as you increase speed and consequently has a bearing on the indicated speed on the dial because the torque on the spring increases proportionately to the increase in actual speed. To calibrate a "perfect" spring torque which will provide a 100% accurate speed indication is practically impossible, hence the newer cars have a digital speed reading dial which is considerably more accurate.
You could open the cluster and re-position the needle so that the return spring just barely returns it to the little stop post fitted into the dial face. This may reduce the inaccuracy to 5mph at 70mph, but it will never be 100%.
A replacement cluster will not be any better, or I should say, there is no guarantee that it will be better, since it too is not 100% accurate.
You could fit a digital speedo, but that will involve considerable expense and electronics conversion. Possibly your best bet other than replacing the speed sensor on the diff to ascertain where the inaccuracy originates is simply to use your GPS to cross check your speed.
The speed sensor on the diff works on an induction pulse. The sensor is in effect similar in operation to the charger of a powered tooth brush charging system. The sensor has two arms which straddle a multi-bladed disc fitted vertically to the right side axle output. As the car rolls, the blades pass between the two arms of the sensor and set up a magnetic field which initiates an electrical pulse in the sensor which is transmitted to the DME, and subsequently to the speedo. If the sensor is faulty, or simply excessively gunged up by old diff oil and metallic wear particles, it may decrease the effectiveness of it's performance. Possibly a diff oil change is required to improve the performance of the ability of the sensor to read and transmit the pulses. If you have never done a diff oil change, this may be a good and reasonable excuse to do it and you can remove the sensor at the same time to give it a thorough cleaning.... :wink:

In the pic below the sensor is the black / white item with wires attached....

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Re: speedo reading a bit slow

Post by peter2b » Thu 20 Aug, 2015 14:29

I think that chap up north was cought doing 157 mph in his m5 needs his speedo checked
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motco
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Re: speedo reading a bit slow

Post by motco » Thu 20 Aug, 2015 14:52

Thanks Southernboy, it's interesting to know where the sensor is as I'd have looked near the gearbox tailshaft otherwise. In case you haven't seen my thread on the EEP_1 problem, I had a flat battery during a period of non-use and on recharging it the odometer began to show the legend EEP_1 and the fuel, tacho, and temperature gauges ceased to operate. Once warm, after a run and restart, the gauges work again. Allowed to stand and cool and they fail again. My suspicions regarding speedo accuracy date to this event. Try as I may I cannot find a fix for this problem nor anyone who does either except a new cluster. There's mention of a coding plug but that is about all. Earlier BMWs had a (fallible) rechargeable cell in the cluster in order to hold data when the car is stopped, but nobody says the Z3 has one.

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Re: speedo reading a bit slow

Post by littlefeller » Fri 21 Aug, 2015 15:52

Mike Fishwick wrote:40 mph at 2000 rpm works out at 20 mph/1000 rpm, which gives 100 mph at 5000 rpm in fourth gear. This is the same as my 2.8, so sounds normal.
As you can see, the Z3 is quite low geared - probably to please the Americans, who desire the social kudos of having a five-speed manual box, but want to start off in second gear, and go almost everywhere in fifth, as if it were an auto box.

It's not necessarily a bad thing though, as under 'real world' conditions this gives the Z3 a lot of flexibility and enviable in-gear accelleration ability. For example, when compared to the 2.5 litre Porsche Boxster of the same period, the 2.8 has far better 'foot down' performance.

An Autocar road test of September 1997 recorded 50 to 90 mph times in top gear of 8.8 seconds for the Z3, and 12.3 for the Boxster -almost 4 seconds at 90 mph putting the Z3 almost 100 yards further down the road! Differences of the same proportion were found throughout all speed ranges in all three upper gears.

What this means is that in overtaking situations where the Z3 can simply be squirted in top gear, the Boxster driver would have to drop a gear - sometimes two gears - to keep up. This was why Porsche brought out the 3 litre Boxster S, but BMW then produced the 3 litre Z3 . . .

In fifth gear I get 25 mph/1000 rpm, which gives 50 mph at 2000 rpm etc. leading to 137 mph at 5500 rpm. All these speeds are approximate, as both the speedometer and tachometer have their own built-in errors. While the speedometer is digital from the final drive to the digital-to-analogue converter which feeds the speedometer, it is still subject to the usual errors of all such moving-coil instruments, in addition to that caused by changes in the circumference of the rear tyres due to wear, or the fitting of larger-size tyres for cosmetic purposes.

In terms of basic accuracy, athough the UK legal tolerance for a speedometer is +10% BMW state that their speedometers should be within +5%.
is not the signal from the speed sensor analogue? where is the converter situated? do the clocks run from analogue signal or digital? I know this has nothing to do with this thread, but it does link nicely to the other thread for altering the speedo.

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Re: speedo reading a bit slow

Post by Robert T » Sat 22 Aug, 2015 00:34

The speed is measured digitally, as a series of pulses generated by a pickup on the diff (optical if memory serves). The car uses the digital speed output for a number of things. The digital reading is converted to analogue by a DAC in the instrument cluster. You can get the digital reading in km/h from the odometer self-test mode. The other gauges work similarly, and again you can get the digital value from the odometer self-test mode.

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Re: speedo reading a bit slow

Post by littlefeller » Sat 22 Aug, 2015 03:17

thanks rob, that is interesting, most speed sensors are simply magnets and coils, its not so easy to adjust an analogue wave than a digital signal. :)

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Re: speedo reading a bit slow

Post by Mike Fishwick » Sat 22 Aug, 2015 08:25

The Z3 speed sensor is also magnetic, so also acts as a magnetic drain plug would! It uses two wires - earth, and a sensor wire from the speedometer and trip computer etc. The two parts of the sensor are intermittently covered by steel vanes mounted on the diff cage, which provide the switching medium.
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Re: speedo reading a bit slow

Post by littlefeller » Sat 22 Aug, 2015 08:43

so its an on off signal or square wave and not a sin wave?

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Re: speedo reading a bit slow

Post by Mike Fishwick » Sat 22 Aug, 2015 09:33

Depends on the type of sensor - if it were a magnetic reed switch it would be a negative-going square wave signal, but with the frequency involved would be more likely to use a coil, with current induced by the intermittent exposure to the magnet, so it could be a rough positive-going sine wave - any experts around? I keep meaning to put my oscillosope on it, but have never got round to doing so.
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Re: speedo reading a bit slow

Post by littlefeller » Sat 22 Aug, 2015 09:40

is the cluster fed the raw signal or is it converted to digital before then? does the signal to the cluster not come via the ecu? or does it go from the cluster to the ecu? or maybe the ecu doesn't need the signal at all, but then what if your z is an auto? just trying to figure where best to intercept the signal

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Re: speedo reading a bit slow

Post by Mike Fishwick » Sat 22 Aug, 2015 12:16

No - the wiring diagram shows that the speed signal goes directly to the speedometer, where the DAC feeds the analogue instrument, and the digital signal feeds the mileage counters and trip computer. It must also feed into the ECU memory to log any BMW dealer servicing activities and maximum rpm etc, which may be handy for BMW in case of warranty claims.
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Re: speedo reading a bit slow

Post by littlefeller » Sat 22 Aug, 2015 14:42

been doing some digging, seems it is a reed valve and uses this vanned spider.
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if so then it would be square and could be adjusted to change the speedo v wheel rotation, or just change the number of vanes

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Re: speedo reading a bit slow

Post by motco » Sat 22 Aug, 2015 16:43

The speedo in my Westfield is wildly wrong (60mph on the dial is really a road speed of 90mph) so rather than spend money on having it recalibrated I bought a cheap bicycle speedo from Halfords and set it to the rolling circumference of the wheels. It reads happily to 100mph but has a slow sampling rate so it takes a second or two to catch up with rapid acceleration. It's triggered by a Hall-effect switch stimulated by a passing magnet on the o/s drive shaft.

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Re: speedo reading a bit slow

Post by littlefeller » Sat 22 Aug, 2015 19:33

now that's an even simpler solution :lol: 8-)

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Re: speedo reading a bit slow

Post by ralft01 » Tue 20 Oct, 2015 23:41

I know that this is a curve and not really on the topic, but I would really appreciate help with my latest Z project. I acquired an old (2002) Pioneer radio with built in Sat Nav and I want to fit it in place of the aftermarket unit currently in situ.
The Pioneer as it is works perfectly as a radio and a CD player, but to use the satnav function I have to connect one of the wires to the speed impulse wire. Does anyone have any thoughts about how I could/should do this? I know it's a crazy idea, especially since the Nav disc is 2001 vintage and is not supported or updated, but I like the idea of an original system.
Also, there is a warning in the very limited manual that attached speakers must be high power to deal with the load or they may burst into flames. I have no Idea what the speakers are like, so any thoughts on that would be welcome.

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Robert T
Site Admin
Joined: Mon 12 Jun, 2006 11:35
Posts: 9983

  Z3 roadster 1.9
Location: Cheshire

Re: speedo reading a bit slow

Post by Robert T » Wed 21 Oct, 2015 14:24

@ralft01 - take a look at this thread, with reference to the GAL wire: http://www.zroadster.net/forum/viewtopi ... &hilit=gal

If you have more questions regarding this, please can you start a new thread.

Cheers R.
Arctic Silver '99 Z3 1.9 & Black '59 Frogeye 1275cc
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usernothername
Joined: Mon 05 Dec, 2016 09:37
Posts: 46

  Z3 roadster 2.2i

Re: speedo reading a bit slow

Post by usernothername » Thu 08 Dec, 2016 23:52

motco wrote: fuel, tacho, and temperature gauges ceased to operate. Once warm, after a run and restart, the gauges work again. Allowed to stand and cool and they fail again. My suspicions regarding speedo accuracy date to this event. Try as I may I cannot find a fix for this problem nor anyone who does either except a new cluster.
Hi motco, did you ever find a solution to this? I have the exact same symptoms, and also have a 2.2. When very cold, gauges dead, EEP2 error, fine when warm. Also my speedo is 10mph out at 70/80mph, increasing variance with increasing speed.
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motco
Joined: Tue 18 Aug, 2009 19:12
Posts: 634

  Z3 roadster 2.2i

Re: speedo reading a bit slow

Post by motco » Fri 09 Dec, 2016 14:29

See you other post usernothername :D

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