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MG MGA - Speedo recal for 5 speed tranny

Guys,
I've seen several mentions made of re-calibrating a speedometer for a 5 speed transmission. Can someone provide more information about why and how re-calibration is done? What is the error that occurrs (too high or too low)and how big an error? I have a 5 speed that I am planning to install.
Thanks,
Gerry
G T Foster

GT
A) Effective rolling radius of rear tires + final drive ratio determines how many turns of propshaft per mile.
B) Speedo drive ratio in gbx determines how A) is modified to yield speedo cable turns per mile.
C) Speedo fitted determines how B) is displayed. Speedos have a 3 or 4 digit number on the face that is turns per mile or at 60mph, so far as the instrument is concerned.

All else staying constant, if you change the gbx, you are changing B) only, so you have to introduce a correction factor, either by changing the speedo drive ratio in the gearbox, or by adding a conversion unit between the gearbox and the speedo. Or, you might be able to find a speedo with different TPM to suit, or have the speedo recalibrated.

Be careful if somebody says "I have X gbx and did Y", because X gbx may have alternate drive ratios available. Also you may have a different speedo, tires, etc. than he has.

FRM
FR Millmore

This thread was discussed on 16/05/2011

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