Overdrive speedometer adjustment

[december2007]

I have an '80 B which has an overdrive unit out of an older MG. It is one for a 76 and older. My speedo does not work correctly now. What can I do to get it correct? Seems like everyone talks about recalibrating my existing speedo. Could I not change out the speedometer from an older B? Thanks.
Steve Ray

[amgba]

Dear Steve,

The earlier B speedo unit is a different size and look as well as calibration.  You could probably get it to fit, but it won't look match the rest of the gauges on the instrumnet panel.  That's why everyone talks about recalibration of the existing unit as the way to go.

Truth is, if you've changed the tire size from stock the speedometer is not reading accurately anymore and should be recalibrated to match the car.  As an example, if you are using 185/70-14 tires vs. the original 155/78-14 tires, this amounts to about a 5% difference in the reading, or seeing 57mph on the speedomoter at an actual speed of 60mph.   This would be a good oportunity to correct the calibration and refurbrish the workings.

I hope that helps you.  Good luck.

Art