Higher gear ratio for manual

[Didg]

My Surf has a manual gearbox and I'd like to up the final drive a bit without changing wheels / tyres.
Would I have to change both front and rear diffs or is it possible to change the centre diff?

[slobodan]

Most vehicles have a final drive of 1:1
This is lowered slightly with an overdrive.
If you want to change the final drive characteristics then you will have to change the diff ratios (both)
Why not change the wheels and/or tyres?

[Didg]

I have recently bought a new set of BF Goodrich Allterain TA's, which are very good btw.
Otherwise the larger tyre option would make sence.