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MG MGB GT V8 Factory Originals Technical - MGC Rear End Ratio

I have found a MGC that a guy is willing to sell, however, he does not know which differential gear ratio it has. Is there a way to determine this by inspection, car i.d.#, etc?

David Atherton

Since the axle could have been changed for anything at any time in its life, and even the diff inside the casing, the car ID is no good to you. You could drive a measured mile, but the speedo could have been changed as well. Probably the only way is to put sufficient revolutions on the input shaft and count the turns on one wheel up in the air with the other down on the ground. Remember to divide the output turns by two before calculating the ratio.
Paul Hunt

Or take the cover off the diff and read the numbers stamped onto the side of the crownwheel?????
chris cooper


You should be able to use this formula subject to speedo and rev counter being correct

MPH = driven wheel circumferance x 60 x rpm
5280 x gear ratio x final drive ratio
dwc can be obtained from most tyre web sites for an appox value.


Thanks guys. David
David Atherton

I had a look on the archive for info I posted on the subject ages ago. Couldn't find it, but did find this from Roger Parker...looks to be exactly what you need?

Posted 13 June 1999 at 21:55:31 UK time
Roger Parker Tamworth

Drain the oil, remove the rear plate and count the teeth on the pinion and crown wheel gears.

The 3.07 to 1 has 14 and 43 teeth.

The 3.9 to 1 has 11 and 43 teeth.

The 3.7 to 1 has 10 and 37 teeth.

I can't remember the 3.31 to 1 ratio but suggest that 11 and 37 will possibly be close.

Whilst the cover is off inspect for wear of gear teeth and frighten yourself!! It is also worth doing other routine checks whilst it is open too.


Pete Green

If you divide the big number by the small number you will get the ratio. I mention this so you donít have to look it up or carry a reference number with you when you check it. I recall reading that the gear tooth count was also stamped on one of the axel tubes, but I have never seen it.
George Champion

The ratio is stamped onto the central casing as 11/43 or whatever.

It is usually covered in dirt and rust!
Chris Betson

It's always useful to know the teeth numbers as these are the only options made. This provides an easy check if you miscount.

Roger Parker

This thread was discussed between 19/12/2001 and 24/12/2001

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