you could try using a gps..it usually tells you the speed your going aswell as directions etc. thats how i found mine out anyways..
[quote name='pedro said:
205/40 17 showing 60ks on speedo = 57ks on roadside speed radar readout.
215/45 17 showing 60ks on speedo=54ks on same roadside speed radar.
So you should be getting a less accurate reading if you have gone up in tyre size/ratio.
Makes you wonder what it would show if on original 16's,might have to try it one day.
less accurate?? i was told it would be more accurate with bigger wheels/tires??
Im still on the original 16s...10ks out at 100km