Ginge !

iTrader: (1)

OK if i wanted to raise the top speed vs rpm on a impreza whats the best ration diff to use ?

also would centre diffs make any difference either ???

i currently got a sti V3 so the box gear ratios are fine, just now wondering about the centre diff and main diffs too

dunx

iTrader: (3)

No expert, but front and rear diffs need the same overall gearing, and then there's the drop gears too ! Centre diff just splits front to rear, IIRC.

The lower the numerical value of the diff. ratio the higher your top speed i.e. 3.9 vs 4.4, where a 4.4 will give much quicker acceleration.

Someone else will be able to quote a site on t'internet !

dunx

Ginge !

iTrader: (1)

that was how i thought it would be too, i know about the front and rear also as i need a new gearbox so thinking of swopping the diffs too

STI_Baly

iTrader: (23)

So many possibilities, so i suggest you use the top speed calculator on this website>>>

Manual transmission ratio chart

The speed calculator is at the very top of the page.

All the diff and gear ratios are also listed here so compare your transmission code to get accurate figures.

Good Luck and Have Fun

dunx

iTrader: (3)

harvey

iTrader: (48)

If you have an STi 3 then O/E transmission is 4.44 diffs front and rear and your centre diff has a 1.0 : 1 drop gear. 4.11 diffs front and rear will give a higher top speed.
3.9 diff will give an even higher top speed again but the gearing may be too tall for your engine subject to what power it is developing.
4.11 diffs, front and rear are quite common.
3.9 front diffs are common but 3.9 rear diffs are far less common.
Almost all UK cars have 3.9 front diffs but the rear diff is 3.545. This is because these cars run a 1.1 : 1 centre diff.
Centre diffs are easy to change but TY752 will not fit with TY754 centre diff and vice versa.
Drop gears can also be changed.