Hi,
Can anyone help me to identify my diff? It is a 7.5" open diff, possibly from a Granada but I'm not sure. I'm 99% sure that it is a 3.92:1 ratio. The numbers are as follows:

Tag: 88 GG JB 392
Casting: 85-GG-4033-BA

Note that the tag might also be 85 instead of 88 but it did look like 88 so that's what I've written. I've been searching the forum and come across various threads related to diff codes but nothing matches my codes.

What I'm trying to figure out is what my options are to move to a 3.62, 3.38 or lower (higher?) ratio but I've only just realised that it is a 7.5" diff and not a 7". I have lobro driveshafts and a custom propshaft and don't want to change any of that so ideally I'd like to find a 7.5" diff with lower ratio or worst case use a 7" diff and space out the upper mount but I need to be sure that everything else will fit as before, especially the driveshafts and propshaft. I have read that the pinion flange is farther forward on the 7.5" diff but not sure.

I want to be able to cruise in 5th gear at lower RPM to improve long range fuel economy and to make better use of the lower gear ratios. At the moment my speeds in gears at 5500RPM are as follows:

1st - 29 MPH
2nd - 46MPH
3rd - 70 MPH
4th - 97 MPG
5th - 117 MPH

Just a bit on the short side. A 3.62 ratio would be better or even a 3.38.

Comments/advice most welcome as always.

Thanks,
Craig.

hello craig, i have a 7.5 (3.62 ratio) inch diff with 100mm diameter output shafts. Contact me if you are interested

The 392 part of the upper code line identifies your diff as a 3.92 diff.
Which gearbox are you using? Type 9 or LT (Rover one), if Typer 9 then a V6 box will help, or a close ratio gear set.
Most Rover V8 engines because of the lower rev range need a higher diff and as i understand the 3.38 is the diff of choice. The Rover has the torque to pull through the longer diff.
The 3.6 will be an improvement.

Hi guys,
Thanks for your replies.

I am running a Rover LT77 gearbox from a Vitesse so my 4th gear ratio is 1:1 and 5th is either 0.833:1 or 0.792:1.

Yes I expected 3.92:1 to be a bit low for the V8 but it was what I had and got it for free so couldn't complain. I had it out for a decent run the other weekend and found myself just using 5th gear all the time which confirmed my suspicions that it was too low geared. Very relaxing in terms of gear changes but a bit buzzy at cruising speeds and not quite in character with a "sports car".

What I would appreciate guidance on is what vehicles to look for which might have a 3.38 or 3.36 or similar ratio diff in the 7.5" size or whether dropping to a 7" diff is worth looking at. I did read that the 7.5" diff is better suited to the Rover V8 due to extra webbing and strength but not sure if this is true. Seems logical though... The diff codes sticky in this forum doesn't cover the Granada much which I think is a more likely donor for a 7.5" diff than the Sierra. I was just hoping that there would be a chart somewhere that would list Granada model/year by diff type and ratio.

I did find something which suggested a 2.9 V6 Granada should have a 3.36:1 diff - anyone know if this is correct? The other option seemed to be a 3.6:1 diff from a 4WD Granada Cosworth.

t16turbotone, I might be interested in your diff if I can't find a higher ratio diff and ideally one more locally so I can avoid postage charges. If you want to let me know what sort of money you are looking for (either post or PM) then that will give me food for thought.

Thanks again both,
Craig.

you get a 7.5" 3.36 diff in the 2.5TD cars (scorpio's I think)
but it could be in other cars as well...

they should have an 'A' axle code on the VIN plate if that helps

[Edited on 6/10/09 by mcerd1]

this may help + look in my photo archives

No1 and as far as a know are correct