HSV.ETA

What would be the best diff ratio for general driving and some drag racing work? I have a 94 EF Fairmont, six cylinder, with extractors, exhaust and a couple more mods soon to come (cam, computer mods) and its an auto. I have heard about 3.5 and 3.9 to 1 ratios going into E series cars, what cars does these two ratios originally come from (i think the 3.5 are in XR's)?

Thanks for your help

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1965 XP Falcon sedan (soon to be a 250 crossflow and T5)
1986 XF Spac Sedan, EFI, T5, lowered, with a few more mods to come.

EFJase

3.45s from XRs are pretty much 'middle' gears. Reasonabll performance, with reasonable fuel economy.

Start getting higher than that and it swings more toward performance, rather than economy.

If u have a Auto, 3.7 gears are ment to be the best suited if u want up and go, ands doint mind higher rpm on the freeway. 3.9s for a 5spd.

To work out new rpm at, say 100kms, divide your current reading by the diff ration u got. Then multiply it by the one u want.

ie. 3.08 gears to 3.7 gears
3.08 sits at 2400rpm at 100km/h
2400rpm/ 3.08=779.22
779.22 x3.7=2883.1rpm @100km/h with 3.7 gears

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EFJase

And i just made up that original rpm figure aswell

HSV.ETA

Thanks for that, what cars have 3.7 ratio in them? Can I simply change the centre or do I have to change the whole diff? A friend reckons that it might be possible to put the centre from an XF or similar, does anyone know if that is true?

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1965 XP Falcon sedan (soon to be a 250 crossflow and T5)
1986 XF Spac Sedan, EFI, T5, lowered, with a few more mods to come.

EFJase

I wouldnt have a clue where the 3.7s come from.
Ring round the local diff shops. Should just be able to swap the gears.

And i dont know about XF diffs, but pintara, and i think VL diff gears fit with a little pissing around

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