When my speedo is showing 90MPH and I reset the average speed part of the Computer, the average speed shows 86MPH. Do they use different measures?<br>Can I use the revs to calculate real speed.<p>This is important as I am always driving at the speed limit on the freeway/motorway and wish to know how much extra I have before running the risk of a ban.<p>

If your speedo is off, the RPMs will be equally off because they are directly linked to the wheel rotation. Personally I have used a GPS receiver to measure current speed. The GPS is said to be within 1mph in this case. Thus I found the speedo to be up to 3mph SLOWER than actual on new Dunlops with correct pressure. This was the case up to 130mph. In contrast, in Europe, the speedo was always 4km/h FASTER than real, because it's the law.<p>- Stan<br>