speedo reading fast..

[jbh325i]

strange one here but my speedo is reading majorly fast.. i was coming through one of these villages tonight that has one of these speed sensor signs up. according to my speedo i was doing 40mph but according to the sign i was doing 32mph.. now i though maybe the sign was reading false.. but then i went passed another one in a different village and got the same result.. my cars running on the standard 15" steel wheels so cant see that being a problem and I'm sure the profile of the tyre wouldn't throw the speedo out by 8mph.. I'm going to put the sat nav in the car tomorrow and try it out on a few roads with different speed limits to see what results i get but is there anyway i can alter the reading?

[Doggy]

They're not normally out by that much - mine reads about 3 - 4 mph fast at 70 according to TomTom

[jbh325i]

well thats what i thought. if it was only ut a 2 or 3 mph i could live with it. my old E30 always reads 1mph out which dosent affect me but the pug is taking the piss a bit