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yes you are correct it is 10% out... i.e at 30mpg... it will read 27mph.



But which one is out... or are they both out?



The lower digital reading is the accurate one. Speedometers are allowwed to over-read by up to 10% but can't under-read at all. In practice, manufacturers set them to over-read by the full amount so that there's no chance that they might under-read.



That's interesting... I'd have guessed that, that would be the way they did it but... M'lady got caught speeding on a motorway... the 'speedo' (mph dial) said 92 and the speed gun (thing) also read 92. If the error was constant then surely the speedo should have been reading 83?

Until that point I always assument that the 'speedo' was out and the digital was spot on.

Now I'm just confused.