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I have looked over my diff top, bottom and side and cannot find a stamp anywhere identifying the rear end ratio. I have just placed a deposit on a T5 conversion and need to determine the ratio to finalize the order. So I tried the method of marking the input and output yokes and got a slightly confusing result. It took just under 4 rotations of the input to one rotation of the output. Odd that my factory options for ratio near 4 are 3.70 and 4.11. Everyone wants pictures so here it is. You can see that the tape mark is approaching 12 o'clock on the 4th rotation. It's probably 10 o'clock. I assume a 3:70 would be just shy of 9 o'clock. I should also note the car was originally a 350/300HP with a TH auto trans. I'm not sure at this point how concerned to be or how far to go to investigate. I really don't want to pull the unit back out and get into the assembly if I don't have to. Could this be just a little slip in the posi-trac It certainly doesn't feel like there is any play in it.
What’s the pedigree of the differential? Looks like it was freshly rebuilt. The stampings bearing the ratio and assembly date will be on the bottom of the housing. However, if the unit was rebuilt, no guarantee the original R&P are still in it.
Assuming you have little resistance during the process of turning the pinion yoke, the posi clutches should not have any impact. I’m reasonably sure you have a 3:70 (or 3:73 if aftermarket gears).
I have been over every inch with a magnifying glass and no stampings exist so it could potentially be a replacement housing (odd). But the car has languished for probably over 30 years. I recently acquired and I am doing a frame up rebuild. I took the diff out of the housing, The oil was perfectly clear, the gears look new and there was no evidence of even the slightest seal leaks. So I took a chance, painted it and put it back on the chassis. We'll see. Mostly I just want to make sure I make the right decision with the 5 spd conversion re wide speed vs close speed. I suppose either way I go I can always correct with a rear end gear change.
Rear Diff Ratio Identification - No Stamp
