Tyre Profiles

Simple the width is greater so the profile is lower to result in the same rolling radius.
on an e39 as an example the standard tyre is a 225/60 x 15 this is the same as a 225/55 x 16 a 235/45 x 17 but the rear would be a 255/40 x 17 it gave the cars a more muscular appearance and enhanced traction.
If you are being totally pedantic then the true rear size would be a 265/40 x 17 tyre.
It is the same principle with your M3

Width makes no difference to profile .... the E39 on the front can handle up to 235's, the rears up to 255 comfortably without rolling the arches. The offset can be between 15 and 25 on a PCD of 5*120 and counter bore of 74.1mm (beware only E39 wheels fit E39's because of this despite what many claim as all other BMW wheels have a 72.6mm counter bore except the mini which is 56.1) an E34 or E60 5 series rim will NOT fit an E39 without machining the wheel.

The E46 M3 uses a PCD of 5*120, counter bore of 72.6 and offset of 35 to 45.

Based on the original 15" and 60 profile as fitted to the 5 series E39 the diameter overall is 627mm, if you fit a 16" with 55 profile the diameter overall is 631.9 (an increas of 0.78%), if you then fit a 17" rim with a 45 profile the diameter is 616.3mm (a decrease on the 15" rim of 1.71%) - I don't agree with fitting of the 17" and 40 profile rubber because it gives a diameter of 595.8 (a 4.98% reduction over the 15" resulting in an almost 5mph under read of the speedo at 70 mph) totally unacceptable and shows that those who do this don't know what they are doing (usually those selling replica M sport rims), 40 profile are OK on 18" giving a diameter of 621.2mm (a decrease of 0.98% over the 15" size).

All data courtesy of Rochford tyres ......

Sorry if I bored anyone ......

The rear size will be sized to make sure the rolling radius / diameter fall within 2.5% of designed diameter and make sure the speedo reads correctly (or as near as possible).

I have yet to hear a convincing reason why they fit larger profiles to the front though - I think it may have something to do with the suspension geometry / camber and tread contact area but have no firm confirmation of this from a BMW source .... 

the 40 / 45 / 60 is expressed as a percentage of the WIDTH which is 205 / 225 / 255 etc etc

http://www.chris-longhurst.com/carbibles/tyre_bible.html

so partly correct, what is important anyhow is diameter ...

1 beer too many methinks ....

Edited by Garfield

Garfield wrote: I have yet to hear a convincing reason why they fit larger profiles to the front though - I think it may have something to do with the suspension geometry / camber and tread contact area but have no firm confirmation of this from a BMW source ....

Surely the reason is to keep the rolling radius of the front wheels the same, or almost the same, as that of the rears. The rear tyres are wider than the fronts to give the car a more aggresive stance but mainly to provide more grip to the driven wheels. If you look at the tyre specs for any sports car, from Lotus elise to Ferrari 360 they always have wider tyres on the back with a lower profile.

Edited by Peter Fenwick

I found out about the rolling radius thing and that was why I mentioned that a 235/45 and a 265/40 x 17 is the same diameter, in the final extension a 235/40 and 265/35 x 18 is about as low as you would want a two tonne saloon to go on Britains crap tarmac and still retain circular rims at the end of the day.

It is note worthy that BMW are the ones who spec a 255/40 x 17 rear with a 235/45 x 17 front, that is why they are the cheaper tyre to buy over the 265 which only has a few makes and types where the 255 is everywhere.

For anyone still confused take the rim diameter multiply by 25.4, then the width of the tyre multiplied by the total of the profile so say 265 x 80% add to the rim diamemter and Bobs your aunty by working out the oem rolling diameter you can work out all the upgrades and downgrades that will not throw the speedo out.(and for those who got it already,,,SORRY! not intending you to be taught how to suck eggs)

If the rolling radius is greatly different front to back, it's going to cause the ABS system problems. They are designed to sense differences in rotational speed which is what different a rolling radius would produce.