Finding gearbox diff ratio
 

I have a ppg box going into my project car, and I thought it had a normal UK centre diff, however today I spun the front wheel whilst the car is on axle stands to make sure the ratios in the box are compatible with my rear diff.

It turns out that when I turn my front wheel two revolutions, the rear wheel turns less than one revolution (about 90% of one rev, I'd say).

Now, my estimated 90% is uncomfortably close to 3.545/3.899 (90.9%).

Is there a way to find out what the front diff is without taking the box out to have a look? As it's been rebuilt at some point, I don't trust whatever markings are on the box.

At the moment, my best idea is running the car up on axle stands and counting revs of both the front and rear wheels to see if they match.

Its not the centre diff your worried about its the front diff and drop gears.