Determine front final drive ratio by experiment / testing
 

I recently purchased a gearbox and rear diff from a private seller who claimed that they were removed from the same car. The box code is TY752VB4CA. Having looked up the code, the box should have a final drive ratio of 3.90. However, the rear diff has a final drive ratio of 4.444 (I have measured approximately 4.4 on the workbench by turning the input shaft, which is easy as it is viscous and remains locked).


I suspect that the box is a different ratio to the diff, but it is conceivable that somebody has changed out the front diff in the box at some point. This box should also have a DCCD, however this has been removed: there is no DCCD cable sticking out of the access hatch, and one of the hatch bolts is newer than the others, indicating that it has been opened at some point.


I have made a couple of attempts to measure the final drive ratio of the front diff by carefully turning both stub shafts at the same rate, however this is not easy to do being an open diff, throwing off the accuracy of the measurement. So far I have measured approximately 3.90 or 4.1 depending on how steady my hands are, which are both valid Subaru box ratios!


Both box and diff are currently sitting in the garage not on the car. Does anybody have a clever method of reliably measuring the front diff ratio while the box is off the car, without resorting to stripping the box?

Do you mean the rear diff or the front diff? It is the front diff I need to figure out.