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.

[Montreal914]

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.

Well, you do the math! (IMG:style_emoticons/default/biggrin.gif)

5280 feet in a mile.
12" in a foot.
Tire diameter x pi gives you circumference.
With this you know how many wheel turn you need to achieve 1 mile.
With some calculations you will have the speed of the axle at 60mph.

Now you need to multiply your final drive by your 5th gear ratio
This combined with your axle speed will give you the engine RPM.

Fun exercise! (IMG:style_emoticons/default/smile.gif)
You can calculate this by hand, but Excel would be a lot more practical as you could change the various parameters (tire diameter, ratios, etc.)

I have something like that somewhere as I was playing with final drives and gear ratios... I will try to dig it up but you would want to check it first... (IMG:style_emoticons/default/rolleyes.gif)

[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.

[partwerks]

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.

Tire size I have: https://tiresize.com/tiresizes/225-50R16.htm

[Montreal914]

Tire size = 24.9"
Tire circumference = 24.9" x pi = 78.2"
Every tire rotation gives you 78.2" of travel.

A mile is 5,280'
5,280' per miles x 60 (60 mph) = 316,800' per h, which means 5,280' per minute (to achieve 60 miles in an hour).
5,280' x 12" (12 inch in a foot) = 63,360" traveled per minute.
63,360" (traveled distance in a minute) divided by the travel of each tire rotation 78.2" (tire circumference) = 810.2 tire rotation (of that tire) in a minute to achieve 60 mph.

Forester final drive 4.444 (per provided info) x 0.695 (6th gear) = 3.0885 Which means that the engine turns 3.0885 times faster than the axles (in 6th gear with that final drive).

Now, knowing we have 810.2 axle rotation to achieve to be at 60mph, and knowing the engine rotates 3.0885 faster than the axle, then: 810.2 x 3.0885 = 2501.7 which is your engine RPM.

Using this and changing the various parameters, you should be able to calculate everything. (IMG:style_emoticons/default/smile.gif)

Sorry for the math lesson... (IMG:style_emoticons/default/rolleyes.gif)

Edit: it's not only about the final drive, it is also about the gear ratios. I am not familiar with Suby stuff, but are those final drives shown in your original post using the same gear ratios (5th or 6th gear)?