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Speedometer calibration


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4 hours ago, Pdv said:

Having fitted a MG Midget gearbox to my 13/60 I have read that I will need to recalibrate the speedometer has anybody had this done and if so what is involved and costs please.



By my reckoning I think your original set up gave 1216 turns of the speedo cable for each mile travelled (standard 4.11 ratio diff and tyres) and you can check this by looking at the number printed on the face of the gauge.

Your new gearbox gave 1000 speedo cable turns per mile with its original 3.63 ratio diff but now (assuming tyre size is unchanged) with the 4.11 diff it will give roughly 13% more. This is about 1132 turns per mile so the speedo will only read approximately 7% lower than before... 

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I agree with the last two posts. However, if it does read slow, be careful when driving through cameras as the rules are much tighter now. It is better to have a speedo that reads fast or spot on.

You can also correct it by fitting tyres that reduce the rolling radius of the rear wheels. Depends if you are happy with the ratios and how worn the tyres are.

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