I agree that Sat Nav is the best indicator of speed, I use both a Garmin and Tom Tom simultaneously, and they always agree with each other, 30 m.p.h. on them equates to 32 m.p.h. on the speedo. I never even thought about the effects of tyre wear and pressure on the speedo readings, but I suppose it is a logical consideration. With the Constabulary adopting zero tolerance in order to keep the speed awareness frauds active, we need to be very aware of our speed.