lnferno

I think every Audi that I've owned, the speedo reads ~2 mph high

snagitseven

Also, consider that tire circumference will also affect the speedo and odometer. Even tires spec'd the same size by different brands will have slight variations in circumference. The you have some differences as the tread wears down; as a result, even if the speedo or odometer could be changed, there would be no way over time to maintain precise accuracy.

dicktahoe

And as the tire wears down, and the circumference gets smaller so the MPH reading will get worse (if it reads 75 when your doing 72, you'll now be only doing about 70.5 when it reads 75) and the Odometer error will decrease (if you're now traveling 105 miles when it indicates 100, with worn tires you'll only travel about 102 miles).

feralc

The yokohamas that came with my A7 are 27.4'' in diameter, the full tread depth is 10/32"
So between a new tire and a completely bald tire you will lose 20/32" in diameter
That is 2.28% difference between a brand new and a completely bald tire
This is an extreme example because you don't usually go that far before replacing the tire.
So if the speedometer is reading correctly and the speed is 65mph you will be traveling at around 63.5mph, not a big difference because as I said you usually replace the tire before.

dicktahoe

"So if the speedometer is reading correctly and the speed is 65mph you will be traveling at around 63.5mph, not a big difference because as I said you usually replace the tire before."

Your calculations are the same as mine--reduce the actual speed you are traveling by about 1.5MPH. I just started already being 3MPH slow at 75MPH indicated as is the case with my A6

twentysevenlitres

You need to realise that tyres don't run round - they consistently have a flat spot on the bottom caused by the weight of the car acting on the air pressure held in the tyres. Even at 100mph the centrifugal force isn't sufficent to get rid of the flat spot.
This is designed into cars from the start - some manufacturers do it better than others (Audi are very good).
As the tyre wears, there is barely a variance in speedo reading (from brand new to worn out, the effect on the speedo would be less than 1%)


Thanks, you made me realise my error - I was correct in my assertions about design rules for speedos, but had my over's and unders mixed up - meant over when I said under and vice-versa! I've corrected my original post now (corrections in bold text)
...also included as excerpt from the Australian Design Rules (which are based on UN recommended standards, which most manufacturers now adhere to internationally)