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Discussion Starter #1
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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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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And i just made up that original rpm figure aswell
 

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Discussion Starter #4
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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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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