THM 425 Ring and Pinion

GryphonGTR500 · #1 (**permalink**) 01-20-07, 05:10 PM

Hey guys,

Does anyone know what size ring+pinion the diff in the THM425 transmission uses? I seem to remember reading somewhere that it uses a fairly common GM ring+pinion. I'd like to gear mine even higher, like a ~2.5:1 or even 2.2:1. Weren't some cadillacs geared this high? Would it be swappable?

Thanks,
Tyler

davesdeville · #2 (**permalink**) 01-22-07, 09:12 PM

IIRC they are TH425 specific parts, and not many out there. I could be wrong.

JTraik · #3 (**permalink**) 01-29-07, 01:55 PM

Quote:

Originally Posted by GryphonGTR500

Hey guys,

Does anyone know what size ring+pinion the diff in the THM425 transmission uses? I seem to remember reading somewhere that it uses a fairly common GM ring+pinion. I'd like to gear mine even higher, like a ~2.5:1 or even 2.2:1. Weren't some cadillacs geared this high? Would it be swappable?

Thanks,
Tyler

Hehehe

oh sorry.... yeah theres no aftermarket parts for the differential, you got what you got.

big block fiero · #4 (**permalink**) 02-13-07, 07:28 PM

three gear sets are available, late model 2.73 gears are the steepest.parts are available to replace the drive chain with an overdriven chain.then youll have 2.41 equivitent gears or somewhere in that ballpark. theres a site just for the 425 trannys that talks all about this.

John Muir · #5 (**permalink**) 09-01-07, 06:51 PM

Not sure on that one. Can anyone help me locate the nylon gear that drives the governor on a TH425. I'm in Scotland and the only supplier so far that I have found who will ship it wants $116 to ship a $3.55 part that must weigh less than 4oz.!!! It seems they have never heard of normal US Postal services. Can anyone point me in the right direction?

BRUCE ROE · #6 (**permalink**) 06-17-09, 09:16 PM

If you put a CASPRO POWER DRIVE, reversed into your THM425, it will make the 2.733 dif look like a 2.40. It as a 65:57 ratio chain, instead of the usual 1:1. Bruce Roe CLC advisor #14630

CadVetteStang · #7 (**permalink**) 06-25-09, 01:10 PM

What year model do you have? 71-79 had the 2.73, but 68- 70 had 3.07. If you have a 68-70 Eldo, then you can gear up by swapping to a 71 - 79 diff. Other than that, the overdrive chain drive as mentioned above or taller wheels with taller tires is the only other way to gear it up.