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MG MGB Technical - Speedo Under-reading

I have owned a 68 MGB for about a month now and now know, at my cost having received my first 'speeding' fine, that my speedo is under-reading. How do I go about getting this adjusted, can I do it or is it a job for the professionals?
M J Ryan

Yeah, they all say that!
Seriously, there are speedos with different calibrations, and yours may be wrong. There is a number on the face that is the "turns per mile", 3 or usually 4 digits, like 975 or 1080 or 1250. Don't know what the correct one is for your car, but somebody will. The actual Smith's part number (also printed on)
will correspond to this.
Otherwise, speedo shop.

FR Millmore

Which speedo you should have is determined by which gearbox you have fitted. If someone has changed the gearbox at some time, you may have an incompatibility.
Dave O'Neill 2

Also check the wheels and tyres. 15" wheels is a common upgrade. If you have anything over 70 profile on 15" it will cause the speedo to under read.
You could also re-calibrate, crudely I admit, by using markers on the glass, and a Sat Nav on a level stretch of road. Might prevent another ticket!!
Allan Reeling

Speedo's can go out of calibration due to age. Worn bushings, a weakened magnet, or a weak spring can cause it. Also, if the pointer is installed incorrectly.

There are articles in the archives and internet about Smiths speedo calibration. It can be rather complicated. An experienced shop or possibly Nisonger can correct it.

Mine reads low also and it is the correct one for my tires and transmission. I've never bothered to recalibrate and just live with it.
Robert McCoy

If it's gearing the odometer will be as inaccurate as the speedo (mind you, I don't think you can be done for driving too far, except by one's better half). A 68 should have a 1280, 1020, 1040 or 1060 up to 67, 1000 on rubber bumper cars. Only the first lot had different tpms for OD and non-OD, after that it was the same. Axle ratio will also affect speedo reading, as well as tyre size/profile and gearbox.
PaulH Solihull

This thread was discussed between 22/05/2011 and 27/05/2011

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