Car speedo off by a lot.

Jimmy Garlic

A friend of mine was done for speeding although he swears blind that he was under the limit. I put my sat nav in the car to see what the story is and it turns out that the speedo is reading 8-10 kph lower than it should be. The car is a citroen c4 and the wheels are the standard ones that came with it. What could be wrong with it?

dgt

Poss wrong sized tyres

Topper Harley

A speedo will never be 100% accurate so they're calibrated to err on the side of cation and over read rather than under read.

So basically your friend was caught speeding despite his speedo under reading his speed. Sounds like he definitely was speeding so. It happens.

EDIT: I realise now that his speedo was wrong the other way. I've got nothing for that.

L1011

Digital speedo? My C4's digital one was the most accurate of any car I've ever had, if its off there's likely a fault with the sensor/pickup.

Jimmy Garlic

MYOB wrote: »

Digital speedo? My C4's digital one was the most accurate of any car I've ever had, if its off there's likely a fault with the sensor/pickup.

Yep, it is a digital speedo.

unkel

A speedo indicating less than the actual speed is illegal.

bazz26

Of course the obvious question is, are your friends tyres and/or rims the correct size that the car came from the factory on? Changing either size will through the speedo off as it is calibrated in the factory for those sizes.

You or your friend can test it themselves though via GPS on your smartphone or sat nav. GPS is as about as accurate as you will get a very good idea if the speedo is out by that much. Allow a certain percentage for a margin of error on the speedo though as they by definition an indication and not 100% accurate.