XJ6 & XJ12 Series I, II & III 1968-1992

XJ6 Series 3 rear diferential ratio choices ?

Old 05-31-2019, 12:56 PM
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Default XJ6 Series 3 rear diferential ratio choices ?

Hi everyone !
I'm about to break down and finally start some of the major suspension restoration / repair projects on my 1985 XJ6 VP ( road salt corrosion mainly, gotta love it ! ). I already have two replacement 2.88 rear end assembles from which to make one good checked / overhauled unit but just saw a used pumpkin from a 1983 Euro XJ6 with 3.058:1 gears. I have also seen some used positive lock units as well. Would it be worthwhile to change to a different gear ratio ? I plan on keeping the 4.2 XK engine but MIGHT change the transmission to a GM overdrive unit later on. The add states that ( with the stock transmission ) it will give more get up and go than the 2.88 without killing fuel economy. What does everyone here think ? Would I be better off just waiting until I do swap transmissions ( if ever ) and maybe go for a positive lock unit instead ? Thanks for any advice / ideas !

Old 05-31-2019, 04:31 PM
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I'm not an expert but:

I can't imagine the 4.2 stock engine developing so much torque you would need a locking differential.

I have an XJ6 that is all sock and another one with 4-speed automatic. Adding that extra gear makes all the difference in the world.

I know in other parts of the world gas is much more expensive but how much will you drive the car that fuel costs are really something to worry about? A couple years ago we drove a 1972 Cadillac form Ohio to California. 12 mpg. But boy was it fun.

If it was me I would go with the stock ratio and install a 4-speed automatic. Not an overdrive but 4 gears with 1:1 in top gear. That will transform the car in the best way possible and it will still look stock.

Old 05-31-2019, 08:08 PM
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I am pretty sure it deeper gears will cut into your fuel economy. Two reasons: 1) The engine will be turning more RPMs which consumes more fuel. 2) The car will be more peppy and fun to drive, and you won't be able to resist putting your foot into it! (and who can blame you?!)

Get the 4 speed transmission... The deeper 1st and 2nd gears will give you the performance you want and the top gear will give you the economy you want. It's the best of both worlds.
Old 05-31-2019, 08:16 PM
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Originally Posted by kudzu View Post
but just saw a used pumpkin from a 1983 Euro XJ6 with 3.058:1 gears. I have also seen some used positive lock units as well. Would it be worthwhile to change to a different gear ratio ?
Going from 2.88 to 3.06 isn't all that much of a change but, as long as you're gonna have everything apart, I say 'go for it'. The car will be a bit more peppy; fuel economy should suffer too much. You'll have to correct some speedometer error, though.

I went from 2.88 to 3.31 in my V12 car and am delighted with the improvement and response. Of course the RPM is a lot higher and it cost me 2-3 mpg at highway speed....but these days I seldom do road trips so it isn't an issue for me.

Old 05-31-2019, 08:48 PM
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If you never want to touch it again install the 3:08. It will be an incremental upgrade that will offer a little bit more quickness that will be noticed. At moderate highway speeds the difference in RPM will be minimal.

If you plan on installing a 4-speed in the future definitely do the 3.31. This ratio with a 4-speed overdrive transmission like a 4L60e or 700 R4 is damn near perfect.
Old 06-01-2019, 06:33 AM
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What four speed auto transmissions will bolt right up to a 4.2? Any drive shaft length issues to deal with?
Old 06-01-2019, 06:49 AM
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I swapped in a 3.31 into an 1985 XJ6, and you can hardly tell the difference in acceleration. These were never fast cars.

I have a V12 VDP with 3.31 and it makes a big difference.

The 3.07 is sought after by the E Type guys. You could maybe sell if for $1000.

John's cars have the kits to put a T700 in. I did one years ago, with 4.09 diff from a 2.8 litre XJ6. It really went good, and still rev'd 600 RPM less on the highway. To me that was max improvement in both power and economy. It was not as smooth though, with some vibrations entering the cabin, likely due to the hockey puck rear mount..

Good luck,
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Old 06-01-2019, 05:16 PM
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Thanks for all the advice & ideas, I think for now I'll just go with the best testing 2.88 pumpkin ( maybe new seals, haven't decided yet ) of those I have, then swap to a 3.31 if / when I change the transmission. That way I'll only have to deal with speedometer issues once. If anyone is looking for the 3.058, Pauls Jag in Florida has it on their E-bay store page.

Old 06-03-2019, 02:46 PM
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My LT! and 4l60 e handles the 2.88 with alacrity. Loafs at 70 in Od, but a throttle stab gets results with no need to down shift...

It does use the GM "hockey puck" rear munt. No vibration issues. I did go to a one piece drive shaft. Odd center joint seems superfluous..

At times, a mystery vibration can be fixed by "clocking" the torque converter.

Old 06-04-2019, 08:38 PM
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i use a 3.73 powerlok with a 700R4 overdive trans. ,, in overdrive final ratio is 2.60-1,, better for cruisin than 2.88 with 3spd trans!

but it really shines for acclleration and passing,!
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