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ianlinux

Speedometer

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Please delete/lock thread. Realised question raised elsewhere - apologies

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Mine over reads too.

When I'm doing 70 on the speedo my TomTom says I'm doing 65mph. Haven't checked the sat nav against the speedo at 30mph though but imagine the error reduces as you reduce speed.

Do you have the standard wheels and tyres still?

Craig.

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I'll check the 30 mph speed against the sat nav next time I'm out then and post the results. At least it reads the right way of wrong I guess, if you stick to the gauge you're less likely to get a speeding ticket. :)

When I built my kit car part of the final test it had to go through was a speedo test which was done on a rolling road. The speedo was not allowed to read in excess of the actual speed and the tolerance for under reading was quite large.

Indicated Speed (mph) Min True Speed (mph)
35 26
40 30
45 35
50 39
55 44
60 48
65 53
70 57
This is the same rules that production cars have to stick too so as you can see, there is a lot of error permissible. Glad my iQ speedo isn't reading 70 when I'm only doing an actual 57.

Craig.

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UK regulations state:
"The speedometer must never show an indicated speed less than the actual speed.
For all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph."

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Thanks both. That explains a lot. I know it's better to err on side of caution, but the difference at 30mph did seem a bit too excessive to be practical. We have cars behind us getting really frustrated at these slower speeds, but that could also just be size prejudice :)

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The fact that speedometers legally over-read was why the Association of Chief Police Officers speed camera guidelines for speeds over 30mph gave a leeway of 10% plus 2mph.

For example if a vehicle was doing an indicated 30-35mph in a 30 limit, this wouldn't incur a speeding fine. Whereas if the indicated speed was over 35mph in a 30 limit then a speeding fine was appropriate..

These guidelines were voluntary and not all speed camera partnerships adopted them.

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