Transaxle final drive Options

[partwerks]

If putting in a trans and different final drives, like 3.70, 3.90, 4.11 for example.
How do you know what the engine rpm is running at, for like 60 mph?

Can a later Subaru Forrester, with a final drive be changed out to a taller gear, like 3.70 for example, instead of a 4.11?
Looks like the WRX are higher priced than the Forrester trans.

[DahlbergDiver]

To find a gear ratio to mph calculator, google that phrase and you'll find a ton of them. Here's one: https://purperformance.com/p-29669-rpm-calculator.html
For the Suby trans ratios, I think I can help you out a bit. I just bought a trans from a 2016 Forester from the local junkyard for $300. The WRX trannies are just too costly, even though they have a more desirable final drive. But, if you are cruising down the road at 55mph on stock tires, the difference between the two transmissions is only 250rpm.
I decided that spending the extra money for the WRX trans was not worth a 250rpm difference. So I decided to just run a little bit taller tires in the back to help bring cruising rpm down a bit.
Here's the specs from my research:
2016 Forester: Final Drive 4.444, 6th gear .695
Rpm at 55mph: 2250

2016 WRX: Final drive: 4.111, 6th gear .666
Rpm at 55mph: 2000

These calculations are made using 25.3" tire diameter. (I believe that's stock)
Hope this helps. I only have data for 6th gear, because I was only concerned with cruising rpm when i researched this.

I also looked into getting the gear ratio changed in the Forester trans, but couldn't find anything by web searching. Maybe a trans expert can chime in, but I think that if you got a WRX ring and pinion from eBay, you could put it in the Forester trans. Would be cheaper than buying the whole WRX gearbox. Just a theory.
I personally am going with the stock forester trans and taller tires. IMO not worth fooling around with changing final drive unless you desperately wanna bring down the rpm.