# Re: New cars failing their first MOT

• From: "Mortimer" <me@xxxxxxxxxxx>
• Date: Mon, 18 Jan 2010 21:28:53 -0000

"Harry Bloomfield" <harry.m1byt@xxxxxxxxxxxxxxxxxxxx> wrote in message news:mn.94c97da1920414ab.106911@xxxxxxxxxxxxxxxxxxxxxxx
Mortimer laid this down on his screen :
Am I right that speedometer calibration is not tested -

The MOT station have no means to test them.

Yes, but that's because it's not part of the test. You're putting the cart before the horse: if it was part of the test, they'd be required to have the means of testing.

that a speedo can drift over time and this may never be detected unless you get a speeding

They drift due to tyre wear anyway.

For a typical 195/55R16 tyre, the total diameter is 2*(55/100)*195 + 16*25.4 millimetres, ie 621 mm.

How deep is the tread on a new tyre? I'm guessing about 10 mm.

So the tyre loses about 7.5 mm radius before it becomes illegal, therefore it ends up at 621 - 15 = 606 mm.

So the percentage change in diameter (and hence circumference) is 15/621 = 2.5%.

So if the speedo is calibrated for brand new tyres, it will over-read by 2.5% when the tyres need replacing: 70 with new tyres will be 70*1.025 or approximately 72 mph with barely-legal tyres.

ticket though you are certain that you were within the limit *according to your gauge*.

They generally read high anyway, never low - so unlikely. You can check a speedo by timing yourself over a measured mile or even easier - using a GPS at steady speeds on a reasonably straight bit of road.

On a quiet motorway late at night, when I can actually mantain a constant speed, I've timed myself over 10 miles (as measured by the car's mileometer) driving at a constant 60 and then at a constant 70. This shows that the speedo over-reads with respect to its own mileometer: at 60, I averaged about 56 and at 70 I averaged 67.

Now of course this ignores any error due to tyre wear etc, and simply measures how accurately the speedo gauge reads when fed with pulses from the sensor that also drives the mileometer.

.

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