View Single Post
      05-04-2014, 03:21 AM   #18
Boss330
Major General
Boss330's Avatar
No_Country
1712
Rep
5,108
Posts

Drives: BMW
Join Date: Mar 2012
Location: Earth

iTrader: (0)

Quote:
Originally Posted by M6-Coupe
Quote:
Originally Posted by Boss330 View Post
I was also thinking of the factory set of wheels available. But since the law in EU allows tires to be changed as long as they are within +/-5% of standard rolling circumference, any manufacturer has to make allowances for aftermarket tires being as much as +5% over standard rolling circumference (not diameter BTW, diameter and circumference are two different things you know ).

Every manufacturer does this and depending on the tires that are on the car, and speed range (some have a higher tolerance at higher speeds), the "error" might range from 1% to 10%.

The M5 also has winter tires that are 255/40R19. The 265/40R19 summer tires are 1,17% larger and makes a difference of 1km/h in speedo reading.

The 167-172MPH runs done by M5 owners, have in reality been 155MPH when they have verified true speed on a GPS during their runs AFAIK. I'm pretty sure you can find several posts confirming this
Thanks again... BTW 5% difference in diameter is equal to 5% difference in circumference and cause 5% difference in speed calculation since it's based on the circumference and number of rotations
That's true.

But ideally you don't want to use diameter X 3,14 when calculating the rolling circumference of a tire

A tires rolling circumference is influenced by the contact patch to the road. This creates a "flat spot" on the tire, meaning it's not a true circular shape anymore... AFAIK this means that tires of different dimensions use different "pi" to calculate rolling circumference. At least that's what ETRO (European Tire and Rim Organization) does, and they are "the law" used by the manufacturers and authorities here

The above means that just calculating tolerance based on diameter X 3,14 won't give you the correct % deviation in some cases.
Appreciate 0