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      09-09-2012, 10:59 AM   #3
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mark896's Avatar

Drives: 2011 E92 335is
Join Date: Dec 2011
Location: Alabama

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Originally Posted by C|3R1C
Yes the wheel is turning faster. However, I believe people do not mention it because the sizes are very similar and yield an insignificant difference. Using simple math and PSS 265/35 and 295/30, you can see on tirerack that the revs per mile are 790 and 799 respectively. The different is .3 inches, with the 295 still being smaller. Relatively speaking, its only off by like 1%, so at 80 miles per hour, you lose like 1 mph. Add that to the fact our speedos (not sure about the digital one) are off by like 2 or 3 as it is, the difference the minimal at best. I also remember reading somewhere that our cars don't use wheel rotation to judge speed but something else, but my memory about that is foggy at best lol so I'm probably wrong about that.

Hope this is a good enough explanation

EDIT: Also, I remember another thread on the forum talking about speedo inaccuracy, with people going like 150mph on the dash, but like 146 in reality.
Good enough, thanks much.