I recently bought an ex Japanese 2003 Forester. Unfortunately the previous owner changed the gearbox and failed to match it with the rear diff resulting in the diff being cooked. How can I know the ratio of the current gearbox so I can be able to find the matching rear diff?

The centre diff will be the cooked one.

 

Easiest way to do it is when you've got the centre diff out, rotate the pinion shaft and count how many times this is rotated to a whole rotation of the diff. Once you get to 3 pay attention to the partial rotation for 3.7 or 3.9.

 

Being a forester I'd suspect it's a 4.111:1 or 4.44:1.

 

The most accurate way is to count pinion and ring gear teeth, divide the ring by the pinion count and you'll get your ratio.

From memory 4.111 is 37/9 ring to pinion tooth count.

 

You'd be best to count in this fashion by splitting the box or using an inspection camera.

 

All the best with it.

 

Cheers

 

Bennie

Thanks Bennie, but in my case the rear diff is the one that's cooked and actually making sounds even though I have already disengaged it. Am actually using FWD.

 

Suppose my centre is 4.111:1 or 4.44:1 then what should be my rear diff ratio???

The easiest way to figure out what you have got is, the trans code by the starter. That will give you the trans ratio..

 

If your front diff is 4.11 or 4.44, just match it to the rear diff of the same. Foresters used 1:1 center diffs so same front and rear. Unless the PO used a 1.1:1 center diff, then it could get messy but I THINK the 1.1:1 center diff was a USDM WRX M/T thing only

Thanks Matt167