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Posted by: JRust Feb 25 2016, 10:29 PM

My reason's are 2 fold. 1st my 01 Forrester tranny is either a 4.44 or a 4.10. Found a good deal on an 03 wrx tranny which has 3.90 for gears. I'd like a to drop my RPM's for highway cruising. I always get a little confused when it comes to how the gearing translate's to RPM's. 2nd reason is if I upgrade my motor at some point from my NA 2.5. My forester tranny isn't strong enough for a six or WRX motor. So while I get lower Rpm's now. It's more for down the road possibilities

4.10-3.90 will drop my RPM's by how much? Say at 65 I'm at 3600 rpm's currently.

4.44-3.90 while drop my Rpm's by?

Posted by: matthepcat Feb 26 2016, 12:28 AM

You would need to know your total tire diameter.

http://www.jeepforum.com/forum/attachments/f9/2051689d1429585918t-gear-ratio-help-noobtiregearsize2.jpg

Posted by: jd74914 Feb 26 2016, 01:33 AM

It's just a ratio (no need for tire diameter assuming your vehicle speed @ engine speed is correct) so if you're spinning 3600 rpm currently you'd spin:

Assuming current = 4.1 and new 3.9 >> new engine speed = 3424 rpm
Assuming current = 4.44 and new 3.9 >> new engine speed = 3162 rpm

Posted by: matthepcat Feb 26 2016, 09:18 AM

^^ that works but not exact ^^....but probably good enough

Posted by: 76-914 Feb 26 2016, 09:29 AM

What size tires are you running Jamie? That seems a little high even for 4:11's

Posted by: mgp4591 Feb 26 2016, 09:41 AM

I think about 12% or so...
But that's why I'm going with 3.55s, for the highway cruising. If I need lower ratios, I can always downshift.

Posted by: JRust Feb 26 2016, 05:53 PM

I'm running 225/45's on all 4 corners. I'm guestimating on the RPM's. I was just going off memory there. Just wanted a sense for what it would drop between the tranny's. Was just looking for a rough idea. Looks like I am hoping my current setup is the 4.44. That will give me the biggest drop. I appreciate the input. I'm doing this more for the down the road engine. Than the need for lower RPM's. That & I'm getting it for $650. It uses the same input shafts as mine so that swap will be easy. The read diff is worth some money so I should be able to get a $100 for that. The subie gears locking spool is $200. Then I need to get a rear plate cut. All in all I should be into it for less than a grand installed. Plan to go pick it up tomorrow

Posted by: shoguneagle Feb 27 2016, 12:54 PM

Jaime, for your info and additional information I am running 3.2 Carrera engine backed by Doc Evil 901 transaxle. My gears are A,F,O,X, Flipped H gear.

The final drive ratio of the flipped H is somewhere around 5.89:1 which gives me 2,700/2,800 rams at 65-70 miles an hour. Once I had the basic drive line and ratios I wanted, I started to change the tire series size to give me lower rpm and still handle producing the needed air across/through the engine. Currently running 235/40ZR-17 in rear; 205/45ZR-17; 9 inches and 7 inches wide respectively

Since you are purchasing the trans and it will exchange with very little trouble, the key to the ratios lies in the gear ratios and the possible build you can get out of the Subie gears available that will fit and handle your needs. As if you did not already know this; just have to mention it to make sure my Old Head is still working correctly. The last adjust would be the tire size and what will work under your car. Check a tire outlet that gives the miles per revs and this should approximate the final adjustment. I am trying to remember what mine is: something like revs per miles per hour (maybe I am confusing you but look for information something like that; Tires Outlet???).

Posted by: JRust Feb 27 2016, 06:33 PM

Picked up the tranny. Turns out he already sold the rear diff. Not a huge deal but got the tranny for $600 instead of the original $800 he was asking. I'd already talked him down to $650 so only another $50 off. Still was a great deal for an 03 WRX tranny. Guy even threw in the starter & the stock turbo off his motor. Little shaft play but he threw int he rebuild kit he had for it. Not expecting much out of it but can't argue with free . I just ordered the subie gears locking spool. $200 shipped