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post #1 of 5 (permalink) Old 02-15-04, 02:42 AM Thread Starter
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Diff Ratios

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


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.
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post #2 of 5 (permalink) Old 02-15-04, 02:49 AM
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Re: Diff Ratios

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

TPC Mod/henchman
previous rides;
1st car EA Fairmont Ghia, extractors, exhaust
2nd EF GLi, 'custom' interior, extractors, exhuast
3rd EA Fairmont 5spd, low'd, extractors, exhuast, head
work, 3.45LSD, EL dash, interior+ El ECU
4th EB2 Fairmont, EL dash and interior, BBM, LSD etc etc
5th EB Fairmont Ghia 5L. All leather, lowered, exhaust etc

Last edited by EFJase; 02-15-04 at 02:52 AM.
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post #3 of 5 (permalink) Old 02-15-04, 02:52 AM
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Re: Diff Ratios

And i just made up that original rpm figure aswell
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post #4 of 5 (permalink) Old 02-15-04, 11:16 PM Thread Starter
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Re: Diff Ratios

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?

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.
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post #5 of 5 (permalink) Old 02-16-04, 03:57 AM
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Re: Diff Ratios

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

TPC Mod/henchman
previous rides;
1st car EA Fairmont Ghia, extractors, exhaust
2nd EF GLi, 'custom' interior, extractors, exhuast
3rd EA Fairmont 5spd, low'd, extractors, exhuast, head
work, 3.45LSD, EL dash, interior+ El ECU
4th EB2 Fairmont, EL dash and interior, BBM, LSD etc etc
5th EB Fairmont Ghia 5L. All leather, lowered, exhaust etc
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