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MG MGB Technical - Correct revs at 50 mph??

I have recently acquired a restored Mk1 1968 MGB, I know that the speedo under-reads and now wonder if the rev-counter is under-reading as well. At approx 50 mph my revs are about 5,500 in overdrive fourth. This seems very high to me....I can't imagine that there would be any way I could do 100mph (not that I intend to) as the revs would be out the other side of the red zone!
Does anyone know what my revs should be at about 50mph?
M J Ryan

In over drive, you get about 21 miles per hour per 1,000 engine revolutions per minute. So, about 2,400 rpm should be showing at 50 mph.

The speedometer can be cross checked by either using a GPS system or finding a place that has mile markers and timing how long it takes to drive one mile. The tachometer can be cross checked by using a dwell tach and comparing the test instrument's readings with the dash tach. This latter can be done in the garage and does not require the car to be road driven.

If the dash tach is accurate, and the speedometer is accurate, you have a slipping clutch or an internal transmission problem.

Les
Les Bengtson

Correction: my original post should have read; '...wonder if the rev-counter is over-reading...'(not under-reading)

p.s. thanks for you comments Les, from what you have said, my tachometer must be wildly over-reading....
M J Ryan

At 5500 rpm in O/D fourth you should be doing well over 100 mph. No, your tacho is way out.
Mike Howlett

MJ - Not only is your tach reading very high, the speedometer could also not be reading correctly. It is very easy to get the speedometer calibrated at a speedometer shop (there is nothing special about the speedometer in a MGB) and that would give you something to reference the tach to. Cheers - Dave
David DuBois

There's a timing capacitor in the 67/68ish tachos that goes bad, giving the over-reading you have. The fix is pretty easy if you can solder and is documented somewhere in the archives.
SteveP

This thread was discussed on 21/08/2011

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