its not to be fair, not massively but it does have an effect
<geek mode on>
take a car with 15" wheels and 205/65 tyres, teh overall diameter of a new tyre is
15*25.4 = 381mm
add teh 'height of the tyre (205/100*65) =133.25
=514.25mm
Circumferance of a 'new tyre therfore is Pi(d) =1615.564021805mm
now a tyre has 8mm of tread when new, 1.6mm when worn so a diff of 6.4mm this is doubled as it affects both 'sides' so it become (514.25-12.8)Pi = 1575.351635847
so a difference of 30mm for each and every revolution. or in percentage format
1575.351635847/1615.564021805 *100 = 97.51 % so a difference of 2.49%
<geek mode off>
