Tyre / Wheel size

[Grazza]

Looking at buying a 98 Z3M roadster that says the wheels have been upgraded to 255/40/17 rear and 235/45/17 front. I have been trying to get an understanding of how these vary from the standard wheels and think that the fronts are wider than normal but am unsure if the backs are as well? Can anyone advise and also how this affects the handling of the car? Also if they are wider than standard will they not stick out?

Thanks in advance for your input

[Robert T]

Hi and welcome to the forum.

Assuming that the car has standard Z3M wheels, then it would have left the factory with 245/40/17 rear and 225/45/17 front. An extra 10mm on the width is not likely to affect things much, and will only make the tyres stand 5mm further proud of the rims, so they won't stick out. Running 255s on the rear is fairly common and will add extra grip, 235s on the front is less common and will make the steering slightly heavier. Neither is a reason not to buy the car. The larger rears will cause the speedo to read slightly out, but only about 1 mph! As the speedo is normally set to read about 5% over your actual speed, this is no big deal either.

Cheers R.

[c_w]

Tyres have been upsized, that's all - nothing really to worry about though for fit or handling - but I think visually a 235/45 particularly looks a bit too tall.

[Grazza]

Hi, thanks for the responses. Bit confused, assume from the difference in size to standard spec that they are slightly wider than normal but nothing else? If so why would this affect the speedo, the seller also mentioned that this was the case? The seller also said that the wheels had been changed to accommodate the tyres but they look like standard Z3M wheels, did they come in different sizes?

[Brian H]

The side wall height of the tyre is a percentage of the width, therefore a side wall on a 255/40 will be taller than the side wall on a 245/40 tyre.

Do you have any pics of your wheels?

[Mike123]

If they are standard wheels I recommend changing to stock size tyres at the next change.

It's not vital, but I've tried the larger width on the front, and felt that there was detectable movement in the tyre in hard cornering which I didn't like, so went back to standard.

[petecossie]

I have tyres 235 & 255 Falken tyres fitted to standard Z3M rims on my Coupe and have absolutely no problems with handling. No spacers required and stance looks OK on my car. I'd suggest that provided the car drives OK on a test drive, it would an easy item to change back to standard sizes if you did actually purchase the car.
I did the same change in tyre sizes on the first Z3M Coupe in which I toured France & Scandinavia and had no problems with that car either.
The difference in rolling circumference is between the two different tyre sizes is very small as has been suggested earlier in the thread.

[Mike123]

I would not worry about the difference for the existing tyres, the speedo will be within 1mph at 60mph, the diameter 8 or 9mm difference, which is hardly more than a new/ bald tyre difference.

Petecossie is probably right, maybe it was my imagination, but I would still change to standard sizes next time - BMW Motorsport division don't get it wrong too often (other than the diff mounting!).

[petecossie]

Just checked the rolling circumference on the very informative web site below which I've in the past to check tyre data.
Fronts work out at 27.15mm (1.42%) increase in circumference which means a 4mm increase in wall height, which is very difficult to see even side by side.
Rears work out at 24.15mm (1.27%) increase in circumference which means a 3.8mm increase in wall height.

http://www.carbibles.com/tyre_bible.html

Hope this helps with making an informed decision on buying the car.

[Grazza]

Thanks for all the input, now the proud owner of a Z3M (see other post) and as the tyres are on their last legs need to work out what to do sharpish!

Tyres on car are Eagle F1, been on there at least 4 years (during which time the car has done 4k miles). Rear tyres are nearly at the legal limit, fronts still have some tread. The seller told me that the car originally had split rim wheels but that a previous owner swapped them for the current ones - having prob. uploading pic says 'board attachment quota has been reached'?

To my untrained eye these appear to be the same as most Z3M's I have seen pictures of? How do I tell if they are standard, do they come in different sizes? Can I just put standard tyres on them rather than the larger ones currently fitted? The seller said one advantage of bigger tyres is it gives you more rubber protecting the rims any thoughts on this?

[akirk]

having prob. uploading pic says 'board attachment quota has been reached'?

you need to upload them elsewhere (e.g. photobucket / flickr / etc.) and then embed them in your post...

regards

Alasdair