Checking your diff ratio

Sometimes doubt arises as to the diff ratio currently fitted to a car, as a significant number of them have had their diff ratios changed over the years.  Reasons include

  • Deliberate modification for:
    • competition use (lower ratios) or
    • better cruising (higher ratios)
  • Owners taking the cheap option and fitting a saloon car diff when their original MG unit gave trouble.
  • No option if the original ratio is not available.  For example, there was also a long period when the original 3.9 crownwheel & pinion was not available new for the early banjo axle fitted to the MGB.

(Note:  A higher ratio is one which makes the car go faster at any given engine speed, so a higher ratio has a numerically lower number of diff ratio, for example 3.07:1 is higher gearing than 3.9:1.  And vice versa for lower ratios.)

Here’s how to check out your diff ratio easily if you need to:

  • Jack up one of the rear wheels (doesn’t matter which)
  • Block the other wheels (both directions), put the gearbox into neutral and let the handbrake off
  • Mark the diff pinion flange with a bit of chalk or whatever, with another mark on the diff housing nose next to it.
  • Put similar reference marks on the wheel which is in the air and the adjacent bodywork.
  • Rotate the wheel two turns, counting the drive shaft revolutions as it goes. There will be about 4 turns of the driveshaft* – note which direction the driveshaft is rotating, so you can determine whether it does more or less than 4 turns.
  • The fraction of a turn is what we want – make your best estimate of it, to the nearest “O’clock” is close enough (take 12 O’clock as being when the marks on the diff line up, even if it isn’t straight up)
    * MGA, B, Midget. V8, MGC are closer to 3 turns, T-types closer to 5.

Now for the calculations:

Because we have only one wheel in the air and the other not turning, the crown wheel will be turning only half as far as the wheel, so in the two turns of the wheel the crown wheel will do one full turn.

Because any given model of diff had a number of set ratio options rather than continuous range, it is now only a matter of matching the measurement up to whichever is closest:

A series diffs came in the following ratios:

3.55, 3.727, 3.9, 4.22, 4.55, 4.875 (and others even lower but not much use to MG’s)

B series diffs came in:

3.07, 3.31, 3.7, 3.9, 4.1, 4.3, 4.55, 4.875 and other unmentionable options past 5:1
T-types accept B series banjo ratios 3.9 through to 5+
There was also a 3.58 ratio done for the competitions department.

The difference between even the closest of these readily shows up, corresponding to the following positions of the driveshaft (remember, 12 O’clock is where the two marks on the diff line up, and I’ll assume the wheel was rotated forwards, making the driveshaft go clockwise viewed from the front):

3.55     3 turns then round to between 6 & 7 O’clock

3.7       3 turns then round to betwen 8 & 9 O’clock

3.727   3 turns, then round to 9 O’clock

3.9       3 turns, then round to 11 O’clock

4.1       4 turns then round to 1 O’clock

4.22     4 turns, then round to bewteen 2 & 3 O’clock

4.3       4 turns then round to between 3 & 4 O’clock

4.55     4 turns, then round to between 6 & 7 O’clock

4.875   4 turns then round to between 10 & 11 O’clock

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