Jack one wheel; put a bit of masking tape on the prop shat, then a bit of masking tape on the tyre you've jacked.
Possition yourself so you can see both, then carefully turn the jacked wheel two revolutions and count how many turns the prop-shaft does.
Use the masking tape, its easier to see, and you WILL get yourself cross eyed and loose count tyring to follow studs on the prop-flange!
With one wheel jacked, and one stationary, you will have to make two turns of the jacked wheel to make the diff (hence crown wheel) turn once.
Series diffs have 4.7:1 crown wheels, so prop should turn almost four and three quarter times for two revs of the wheel.
Coiler diffs have 3.5:1 ratio, so prop should turn three ahd a half times for two turns of the wheel.
There are rare alternative ratios, 4.2, & 3.8 I remember seeing advertised in some places, and a cuple of higher ratio ones around 3.2 and 2.8 which I think are SD1 ratio's; but 4.7 & 3.5 are your likly ones; and difference is enough that you should be able to tell quite easily from this test.
But, if you want to be sure, when you have taken it out; count the teeth on the crown wheel and the teeth on the pinion, and work the ratio out.
