UK-POLOS.NET - THE VW Polo Forum

nogwuy wrote: well if you have a greater wheel diameter due to higher profile tyres, then you are travelling a greater distance for each turn of the wheel. therefore your acceloration will be less and the calculation for your speedo will be out .It will say you are going slower than you are in this case, not very desirable in speed camera infested areas (this effect gets greater the faster you go, e.g. it may be 3mph out when travelling at 30mph, but 7mph out when travelling at 70mph) It depends how many % greater the wheel diameter is from standard as to the error % of the speedo.

But your fuel economy and top speed will be increased in this case because at any given engine rpm you will be covering more ground than before (well so long as you are in gear of course ).

I had the opposite effect to this on my mk3 golf gti. i got 16 inch rims to replace the standard 15 inch ones. the original tyres had a profile of 50, but i over compensated for the bigger rims by fitting tyres with a profile of 40 (should have got 45 for the best mach to the original overall diameter) my acceleration was improved, my speedo over-read, fuel economy was down as the speed i was doing at any given rpm was now less. (this was really annoying on motorway driving as the revs were now higher at the same cruising speed, effectivly meaning top speed is reduced)

i changed the wheels again for 17inch rims again with 40 profile tyres, and all was back to as it was before (as the overall diameter was the same as 15 inch rims with 50 profile tyres). i was able to accuratly chech the speedo readings each time as i have a road pilot (camera and accident black spot warning system) which has a gps speedo so i could compare it to the one in the car.

sure all these things are effected to a greater or lesser effect depending on the % differance in the overall diameter of the wheel. But its physics and you cant argue with that.