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      05-03-2014, 05:36 PM   #11
Boss330
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Quote:
Originally Posted by M6-Coupe View Post
Thanks for great explanation.
Now, I agree with the fact that speedo may read a bit high but not that much!and I am not talking about people who change the rims and tires by their own with no attention to ratios and so. I'm talking about BMW wheels installed or offered by factory. Now let's do the some calculations.
You referred to circumference. M5/M6 have standard and optional wheels.
The standard one is 19" and tires are 265/40/19 for front and 295/35/19 for rear. The diameters are 694.5mm front and 689 mm for rear
The optional wheels (343M) are coming with 265/35/20 for front and 295/30/20 for rear. now the diameters are 693mm for front and 685mm for rear. as you can see the difference in circumference between these two sets is 0.5% and not 5%!
You mentioned the calibration for manufacturer of tires and I think Michelin, Good year, Dunlop or Bridgestone, that mostly provide BMW tires are high rated in quality and accuracy.
You also explained about law which makes sense to me but how much? 5%?
this does not make sense to me. overall having a number like 1-2% may be real but 5-10% is too much! OP captured 169 mph over 155 speed limiter. what I am trying to say is that definitely the actual speed limit is not what is claimed by factory and can be something about 160-165 mph.
Anyways I'm engineer and good in math and not lawyer to know the law
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

Last edited by Boss330; 05-03-2014 at 05:43 PM..
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