BlackadderII

I was tootling down the high street the other day and suddenly spotted a mobile scamera

Looked at my PDA and saw 33mph, then looked at my clock and saw 36 - 37 mph, which is right? the PDA or the clock. I've read about people on here saying that clock speeds are a good few mph out but always below actual mph shown on the clock, that would make the PDA rediculously out by say 5mph wouldnt it?

Given the strict mph laws shouldnt manufacturer clocks be a bit more accurate?

DPat

GPS at a constant speed
Speedo by law has to over-read

btw SIAL

Dracoro

Not quite true. By law the speedo must not under-read. As a result manufacturers build in a tolerance which usually means they over-read. by how much depends on the quality of the car I guess. They have to allow for tyre wear etc. as this will affect the speed quoted (but not by much).