Need to know what type of transmission has like a 3.90 FD gear. I have a 4.11 now and my round track Impreza rwd only just spins tires in 3rd gear. 

...can you just swap in a different rear diff w/the 3.9 since you're  RWD? ebay will show ratio....Not 100% sure as it has a weird 1.1:1 center diff, but the '02-'08 (?) WRX or STi should have a 3.9 or 3.54.

Also, the old SVX had a 3.54, if you  find one, but be aware that around '95 the bolt pattern on the pinion flange on the rear diff changed on Subarus - @ least in the USA. The flange can be changed...with the right tools/puller (?).

Edited June 4, 20187 yr by wtdash

Post a little confusing. Are you looking for a transaxle or a rear differential?

rear diff. Im spinning tires coming out of the corners in 3rd gear. either I need a bigger stagger tire, which im trying first, or swap the rear diff. but don't I need to swap transmissions as well?

 

No, you can just swap the rear end. Just don't try to go back to AWD without matching them back.

I've seen several 3.7 LSDs on ebay for around $150.

ima try a 3.90. how much difference yall think that would make?

 

Very little but that's just my opinion.

so a 3.54 will prolly b my best bet?

 

I know nothing about circle track setups, nor your setup....

 

but it sounds like you're running RWD, yes? No front axles connected? If that's the case, then you don't need to worry about changing the transmission.

Subaru r160s come in 4.44, 4.11, 3.90, 3.70 and 3.54. 

Pick your gear calculator, and see what the difference in rpms would be at a given speed and tire size. I don't remember if any EJ-platform Subarus used a 3.7, might only be the EA82ts and such. I know the only Subaru to use a 3.54 is the SVX (edit: nope, used on plenty of WRXs, too). Although it's not uncommon for SVX owners to swap to lower ratios, so you might be able to find a decent used one.

 

Nissans also used r160s, and long-nose r180s are also usually a direct swap (pending axle spline counts), so there might be other ratios available that way.

Edited June 6, 20187 yr by Numbchux

I was suggesting the 3.7 LSDs as a solution because of the lower ratio, and the limited slip to put more power to both wheels.

They seem to be fairly common on later-model WRXs.