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MG MGB Technical - Checking Rec Counter Accuracy

Does anyone have any, not too expensive, ideas on how to check the accuracy of the Rev counter?

Regards Charlie.

Charles Goozee

Another rev counter? By that I mean one as part of an automotive multi-meter. As a crude check you could check the revs at various speeds in a given gear, but that is more dependant on tyre variations. I needed to go the other way when I flipped the needle off the speedo by accident. I knew the rev counter was accurate - or at least agreed with my multi-meter, so it was just a matter of supporting the car safely with one rear wheel off the ground and running it at 1666 rpm in 4th gear to give 30mph and popping the needle back on.

Both 4 synch gearboxes give 18mph per 1000 rpm in 4th and 22 mph in OD 4th, 3 synch gearboxes give 0.1mph less and 0.3 mph more respectively i.e. not enough difference to worry about. You can do the same thing in lower gears to check the calibration at higher revs at sensible road speeds:

1st 5 mph per 1000 rpm
2nd 8 mph
3rd 13 mph
OD 3rd 16 mph
Paul Hunt 2


You can check its accuracy using a stroboscopic lamp including a rev counter together.
For more informations, have a glance at the following URL:
"" and reach the TACHO CALLIBRATION chapter.

If you or a friend have a Dwell meter they normally have a tach built in.
John H

I don't know if gunson still produces the Auto-Ranger, multifunction dwell, tach, voltmeter, but if they do, I can vouch for the acuracy of the unit. Although it has an analog display, it is a digital unit and when I checked it against lab instrumentation, using a function generator and frequency counter, it was dead accurate regardles of what scale I used on the tach. That is what I now use to check how the tach in the car is doing. I have even gone so far as to wire in a plug in the passenger compartment to connect the Auto-Ranger in whenever I want to check the tach. Cheers - Dave
David DuBois

This thread was discussed on 14/09/2007

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