3.45 ratio in an R31 diff?

[Ashh]

I want to bring the ratio down on my R31 diff to 3.45 (which was available in some commodores which apparently used the BW78 but potentially with a slightly different sized crown wheel??)

Does anyone know if the R31/Pintara are 0578 or 0575 gearsets?

Or if they're interchangeable?
There's so much conflicting info out there!

[NME308]

Hi Ashh,
The R31 are the 0575 (M75) gearset.
I am pretty confident the 0575 gears can be fitted to the 0578 (M78) diffs but I think the other way is not so easy... My diff guy says not do-able while I read others out there saying it 'can' be done with some effort.

Cheers,
Jason

[Ashh]

Ofcourse the time I find this out is when I've just shortened my R31 case... Damnit!!

[Ashh]

I might be saved (also for others if they find this). It looks like the commodore VN (which had a 3.45 ratio) used the M75 gears, while the falcon used the M78.




Thanks again for the good info Jason!

[NME308]

A competent diff shop will have the books and can show exactly which ratio's are out there.
There is actually a far greater range for the M75 thanks to early falcons, commodores and valiants etc than there is for the M78 in some regards.

Good to see you have found at least one option already!

Cheers,
Jason

[Ashh]

Ah cool

Turned out I was reading the W55 numbers off my spreadsheet calculator instead of the W58 numbers, which means I'm probably best off with the 3.7, or at a stretch keeping the 4.11 Pintara gearing.

3400RPM @ 110kph... haha at least overtaking will be easy!

[SmokinSquirrel]

E series XR6 Falcons run a 3.45 ratio with LSD
Most older valiant stuff I had was 3.23 and odd shit like that.
Some fairlanes and falcons (XDs etc) ran a 2.9 gearset.
Seen some R31s and VLs with a 3.9 gearset
Cant remember axle codes but wouldn't be too hard to suss out