Sidewall Sizing

[135BANDIT]

Hi

Can anyone tell me what difference if any
a 40 size sidewall compared
to the standard 35 will make ???

Why does BMW fit 40 on the front and 35 on the rear?

JD

[Jeef Beef]

Originally Posted by 135BANDIT

Hi

Can anyone tell me what difference if any
a 40 size sidewall compared
to the standard 35 will make ???

Why does BMW fit 40 on the front and 35 on the rear?

JD

That number represents the profile of the tyre as a percentage of it's width. Therefore, 40 = 40% of the width, 35 = 35% of the width.

Now since the 135i uses a staggered setup (fronts are 215 and rears are 245 in width stock) then obviously the profile number has to be slightly higher on the front tyres (40% of 215 compared to 35% of 245) to achieve approximately the same profile thickness.

Hope that makes sense.

[minijet]

Have a read of this
http://www.tirerack.com/tires/tirete....jsp?techid=46

but from my understanding, the sidewall profile number is actually a percentage of the width.

ie for my rears its 255/35/18
255 mm wide tyres
35% of 255mm =89.25mm which should be the height of the profile of the tyre.

as you can see the profile/height of tyres are not comparable without taking into consideration the width.

Originally Posted by 135BANDIT

Hi

Can anyone tell me what difference if any
a 40 size sidewall compared
to the standard 35 will make ???

Why does BMW fit 40 on the front and 35 on the rear?

JD

[Mikeshot1]

All of the above information is correct. Since the rear tire is larger in width the side wall percentage has to be smaller to remain the same percentage ratio as the front or vice versa. This is all so the tire height or roll length is the same. This is especially important on awd or 4wd vehicles because the transfer case won't like it very much as it will try and make the larger wheel spin faster to keep up with the smaller roll tire. Hope this helps but is all to have the same height tire from the side view. There are tire and rim size calculators to even out your stickers if you want to go larger.

[135BANDIT]

Does this then Alter the speedo - or are the electronic gearboxes able to adjust for tyre differences.

Using an Online calc I note that the revs/mile change as follows

245-35-18 = 815 r/mile

255-35-18 = 806 r/mile

JD

[minijet]

Yes it theoretically does change the speedo speed. but i have changed from 215 f 245 r to 225 f 255r and using my gps to compare it says that at 100km/h on my speedo im actually doing about 98 actually per GPS.

Originally Posted by 135BANDIT

Does this then Alter the speedo - or are the electronic gearboxes able to adjust for tyre differences.

Using an Online calc I note that the revs/mile change as follows

245-35-18 = 815 r/mile

255-35-18 = 806 r/mile

JD

[Jeef Beef]

Originally Posted by 135BANDIT

Does this then Alter the speedo - or are the electronic gearboxes able to adjust for tyre differences.

Using an Online calc I note that the revs/mile change as follows

245-35-18 = 815 r/mile

255-35-18 = 806 r/mile

JD

The difference is negligible.