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MG MGA - Speedo Calibration


I have a Mk2 Overdrive gearbox in my MGA. The car runs 195/60 tyres with a 4.3 diff.

The speedo was reading very high so I had it rebuilt (at huge cost to the management) by a very reputable and experienced company in Melbourne Australia.

They asked me to count the number of turns of the cable over a 20 metre distance (66 feet). It was 17.5. They then calculate the number of turns per mile and gear the speedo accordingly. They have a range of different gear sizes specially made so they can get it fairly close to the required TPM. After waiting ten weeks I get the speedo back and it reads approx ten percent high through a wide range of speeds.

I will ring the company concerned on Monday morning, but assuming for the moment that they have geared it correctly, are there any other variables that could account for this discrepancy??

Brendan
Brendan Sullivan

Brendan, the A.D.R.'s in Australia allow for a 10% variation between actual speed and indicated speed. All speedo's tend to show over not under the speed because the car maker does not want to be sued by a motorist caught speeding by an under reading speedo. This is the reason police pursuit vehicles have the speedo recalibrated and regularly checked. let the gauge repairer know you want the actual speed indicated or use a G.P.S. Ian.
I Hazeldine

I would assume the rules regarding vehicles are similar in Australia, in that the speedo MUST NOT read lower than the true speed. The tolerance is all positive and most manufacturers work towards the high end of the spec. So in other words most car speedos read 5-10% over, a feature easily seen with a satnav.

To answer Your question Brendan, there are not any other variables. Tyre pressure can make a small difference, but barely noticeable on the speedo.

Before contacting the company you should perhaps check the accuracy of your milometer/trip-meter, to see how accurate the speedometer gearing is.

The needle position on the speedo can be slightly adjusted fairly easily (after partial disassemble of the speedo) if that can help. That will depend on how linear the inaccuracy is.
Neil MG

This thread was discussed on 13/12/2014

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