Speedo over-reading? How do I check?

Soldato
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My car is a 17 year old Vauxhall (old enough to drive itself? Who knows). Anyway I suspect the speedometer over-reads by a fairly hefty amount. This is because I can go past those 30 signs that light up if you're speeding at an indicated 35 and still not set them off. Not to mention people behind me getting very impatient with me if I obey the speed limit on my speedo. Is there any good way I can check how much it is over-reading, or even better, a way to recalibrate it?
 
Soldato
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16 Jul 2004
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Do you have a nav device that can display speed? If not, a phone with GPS capability? You might be able to get an application that will display your speed on that, and it will be reasonably accurate.
 

Jez

Jez

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Borrow a portable sat nav such as a tomtom and read the speed off that to check it before you do anything else :)
 
Soldato
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This is quite possible if it's running on lower profile tyres than it should or if it's had a replacement gearbox with a different final drive fitted etc. Calibrating it yourself isn't easy at all, but there are companies that do this.
 
Soldato
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Bristol, Old Blighty
Again, I wasn't even aware such things existed. This thread is starting to make me feel like a luddite.

Well, I'm taking the car to a garage on Monday to have new alternator bushes fitted. Will they be able to do a speedometer check while I'm there?

Oh and I already know the speedo over-reads by 10mph when I'm stationary because that's where the resting pin is. :p
 
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