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)