3.70 Gears

[Laila Berri]

Sothere's talk of taller gears and shorter gears. Am I correct that as the number gets 'larger' the gear gets shorter? So 4.44 is shorted than 3.90? And what does this mean in terms of output to the ground?

So, for example, given a NA EJ25 running 3.90 5MT, would that have slightly lower RPM at 70 in 5th than the same motor running a 3.90 5MT at 70 in 5th (all things being equal, like tire diameter, model, weight, etc.)?

I'm running a NA EJ22 5MT with 3.90 final drive. I've upped my tires from stock 185x70x14s to 27" At's. I've done some calculators online and think to compensate for the increased tired diameter (and maybe weight), I'd need like 4.54 gears to regain "stock" driving characteristics.

To compensate for these taller tires I think I can regain more stock-like driving (e.g. stop to go acceleration) characteristics by going with an Ob transmission (5MT 4.11 final drive), or maybe even 4.44 final drive, although I think those trannies may be higher $$ compared to first gen OB 5MTs (and some have even said first gen legacies ran 4.11 final in their 5MT)...

to clarify, most of what you're mentioning is "final drive gearing" related, not just "gearing" related. 3.7....4.44 is all final drive specific. the gear ratio of the actual gears inside the transmission also make a difference, not just the front and rear diffs that determine the final drive ratios.

it might be the case that all the 3.9 and 4.11 EJ MT's use the same gearing in which case the only change is the final drive of the diffs, but i know across some they did change the gearing too (not just final drive). might want to check the nice rallispec...or whatever chart that is that breaks down all the MT gears and ratios.

maybe it's possible to install a 4.44 front diff in a manual trans, i'm not sure how the ring and pinions shake out between the front diffs but that's a rather big job requiring splitting the transmission case.

To do what you'd want, you'd probably have to keep your transmission and swap the final drive to 4.44. Just swapping transmissions with 4.44 final drive won't cut it, because they change the gearing a little to compensate for it. I'm basing this on some gear ratio spreadsheets someone sent me a long time ago.

I like the way my SVX drives with the fd of 3.54. it cruises at 80mph at about 2100 rpm lol getting the 27.5mpg that it does on avg. if you leave it in 3rd gear, cruising at 80mph is about 3600rpm i think last i checked, so redline of 6500 is a long shot....

Good point. i do have that MT spreadsheet and have also found similar gearing info from the EA82s (for when i was thinking of a 4x4 swap)... But doesn't the final drive calculate with ALL the gears such that cars with the same geared 1-5 but different final drives would have slightly different driving characteristics?

So to compensate for taller tires, should i look at the gearing of cars with stock taller tires, like OB? I think the only difference b/t their 5MT and mine is the final drive. I think my MY 95 had 4.11 as an option, but my rig is with 3.90 final...

My STI has a 3.90 final drive. It feels like it has short gears because its actual gears are shorter. If you put a 4.44 final drive in it, you'd be shifting constantly and probably be hitting 4,000 rpm on the freeway. The SVX is gear limited to 196 I think, while STIs are limited to 165. Some SVXs came with 3.90 as well. The difference is the gear ratios, not the final drive.

As I understood it, there were two years of the Forester XT's with 5MT's that had a 4.444 final drive. So those parts can be ordered here and installed into a number of other 5MT turbo transmissions. The cost is pretty high of course so probably people are getting the stuff used for less from Japan (out of used JDM tranmissions I suppose).

Basically, the gearing inside the transmissions stays pretty close to the same within a small range. The final drive is often changed to make the car feel more/less sporty or to offset a larger/smaller tire/wheel combo.

But it's not that cut and dried - the differences between say 3.9 and 4.111 is REALLY small. The first generation Legacy (90 to 94) had a 4.111 final drive. In '95 they went to a 3.9 final drive. The gear ratio's in the transmission probably changed a bit as well, but I recently put a 4.111 tranny/rear end from a '91 into a '96 with a dead 3.9 tranny. The difference is not even noticeable.

Really - unless you get the transmission gearing AND the diff from something like an Outback, etc where the final drive was actually modified to offset the larger tires then you aren't going to affect much change by changing only the final drive ratio. You have to look at the total package and how the gearing inside the transmission is setup AS WELL as the final drive. The end result is what matters and there's a bunch of ways you can get there. Very tall gears inside the transmission will still make a car slow even with a high final drive ratio, and very short gears will make it fast and with no top-end.

The final drive ratio is chosen mostly for what it will do to first gear - which is nearly always very similar on all transmissions. If you want something that *jumps* off the line (or to correct for larger tires) then you choose a higher number for your rear diff. If you want a more economical and reasonable acceleration then you choose a lower number.

Ahhh. I see it's not so simple. Hmmm, so my first instincts of getting an OB tranny were pretty good. Now to refine and figure if its 1st gen or 2nd gen OB, and how foresters would also factor in. . . AND to complicate it all, i'm interested in an Australian Forester transmission, maybe from an XT, but not positive yet. . .

So to compensate for my taller tires in an otherwise stock 95 3.90 5MT, I'd be best to get the whole package (i.e. tranny and rear diff) from a rig that had a taller tire setup, like an OB or Forester?

I do have the Subaru Manual tranny chart (but can't seem to attach it) and have found gearing from earlier generation manuals.. So I could review the 1-5 gearing of what I think I'm looking at (5MT forester XT) vs. my leggo...

Yeah, that's what I was trying to say before. It's how the whole thing is setup. If SVXs and STIs have the same final drive (for the most part), then how is the SVX gear limited at 196 mph vs. 165 for STIs? Blindly grabbing a setup with the final drive you want may be disappointing. Imagine having a FXT and wanting better gas mileage. You could see an STI 6-speed with 3.90 final drive and think it would help. You'd be wrong because the actual STI gears are shorter.

Am I reading this correctly? The USDM forester is geared identically, save reverse and FD, to my 3.90 FD 5MT. The AUS forester (NA) is geared a touch shorted in 1 and 2. US EA82 is geared way shorter in 2-5 than my 95 legacy...

When you're going back as far as the ea engines you also have to take into consideration the power those engines were putting out. The ea82 only pushes 90hp or something so it NEEDS shorter gears in the transmission to be able to move the car at any decent rate. Acceleration or speed. Then take the RX with the 3.7 FD. It had a turbo so it has the power to go along with a higher FD ratio.

You're correct about the EA trannys having shorter gears. But with the bigger tires on my GL 5th gear actually ends up almost exactly in the same place as it does in my legacy in terms of RPM vs speed. Both have a 3.9 FD. Some of the other gears are a little taller, some are a little shorter, but the gearing in the subaru trannys has changed quite a bit over the years.

And I thought only auto trannys in the US had the 4.44 FD. I've always been under the impression that the OB and fozzy had the same gearbox/FD. The EJ boxes do like to play with 5th gear though, there's a few different ratios for that.

Wow, good read and lots of good info! I can't add much besides this... I have an 08 outback 2.5xt limited with a 5mt and it is 4.44 final drive (it's also for sale btw). So, there are USDM subies with a manual trans and 4.44 FD. Hope I'm not putting out info thats not necessary! Thanks, Spencer

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