IRS and differential options

I am rather ignorant when it comes to the rear end of my XJC. As I am starting to see the distant end of the world i am doing on the front end of the car and engine / transmission (THM700), I am starting to toy with the idea of acquiring a spare IRS to rebuild while the car remains on the road (if it ever gets back there…) for an eventual swap.

I read things about diff ratios, and posi and outboard brakes etc that make me wonder what the “best” options would be for my setup (respect for original is no longer my concern).

There is no shortage of XJ6 and XJ12 and XJS in junkyards around here so I could certainly pick and choose more adequate than stock, if only for the fun of it.

So folks, what poison would you recommend and why?

Thank you.

Lucky you!

Standard ratio 3.31 and PL, unless you’re into drag racing … I’d opt for 3.07 and PL though as they allow for quieter cruising at 75 mph. If you find one please let me know - I might come around just to pick it up …

Good luck


75 XJ6L 4.2 auto (UK spec)

Thanks Jochen, what does “PL” mean? And what wrecked car should I look for?

Ah, Power Lock or Jaguar speak for Posi Traction ?

Yes. Limited slip.
When you‘re looking at the differential there is always a tag with the gear ratio, and if you’re lucky another tag - P/L. And of course you can turn one side and if the other goes the same way it’s a pl.

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I found in another thread:

“3.07 All V12 Series II. All 3.4 & 4.2 Series III.
2.88 All V12 Series III.
LSD fitted to Canadian XJ6 Soverign and V12 VdP.
1982 model year onward all the North American cars had the
2.88 differential.
The 79/80 and 81 cars had the 3.31 ratio (both 6 and V12 cars).”

Now to find the same list for XJS.

And for XJS:

I found:
1976-1985, 3.07:1 or 3.31:1 (1976-1982), or 2.88:1 (1982-1985) .

And all XJS had Limited Slip.

If I understand well, given that i have 4th gear on the THM700 I could go for a high ratio to get great acceleration and a relaxed high speed cruise (although high speed on california roads in only 79mph…).

So 3:31 from a pre-facelift XJS for the limited slip should be a good option. Right?

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The 3.07 must be expensive.
I think my UK 84 has a 3.056 - not pl. And the 3.31 and lower can be found in earlier XJs before your list, but good luck finding one that isn’t open.
I like low rpm so, yes, that should work. Faster in 1st and less busy in 4th if the ratios work out this way - you can calculate if you want. There should be plenty on the forums already.

I also read re XJS:

“1983 saw the release of the 3.6 six cylinder engine with 5 speed gearbox and 3.54 diff ratio. This combination
(4.0L released 1991) greatly improved the fuel consumption for the XJS, and in 1993 the 6.0L V12 was
released with 4 speed auto with final drive 0.7 and 3.54 diff ratio, again improving fuel consumption for the V12.
The V12 4 speed auto and higher diff proved to be the best combination for the XJS, providing both
improved acceleration times and economy - a combination that should have been in the car from the

So a late V12 XJS (plenty of these around in California) could be a good option with the THM700. I wonder if the whole IRS would be an easy graft into the 1975 XJC…

I had an S1 V12, standard with 3.31 and PL. My personal view is any higher ( numerically lower) would detract from performance somewhat.Keep in mind that carbed S1 V12 was among the faster XJ’s, both in acceleration and top speed. Both my E Types had 3.07, but they were hauling way less weight and performed just fine ( better than fine ,really). Which ever way you go, I don’t think it will materially affect fuel consumption.

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Two scenarios springs to mind, Eric…

How much time to you actually spend on accelerating versus cruising - comfortable cruising versus jackrabbit way of getting there. Then there is hilly country; where a suitable diff ratio allows enough power to remain in a higher gear on the uphills.

Engine power is directly related to rpms, and obviously the 2,88 gives higher rpms than the 3,54 at any given road speeds - to talk extremes. And in all this; lower gears for acceleration is readily available, automatic or ‘manual’, at any time…:slight_smile:

Diff ratio remains a matter of personal preference - as expressed in other posts…

xj6 85 Sov Europe (UK/NZ)



Regardless the ratio, I too like low revs, definitely get a later unit that comes with tapered output shaft bearings. A much superior design from the early roller bearings.
LSD is very good if you can find it but not detrimental.
An of course if you find one with outboard brakes would be ideal, albeit you will have to modify your brake lines.

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Wouldn’t be the opposite?


With the 700 transmission the world is your oyster. You can have the improved response and acceleration of 3.31 or 3.54 while still enjoying relaxed highway speed cruising thanks to the OD 4th gear.


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I have 3.07s in my E Type (3.8 Moss) and XJ6 (4.2 auto). My Mk2 has a 3.54 (3.4 all synchro, OD). I think all of them are ideal as they are.
The XJ6 has no trouble turning a 3.07 - came with the car in 1972. FWIW. Paul.

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In the overall scheme of things, final drive ratio only impacts performance in 1st gear and top gear. In between, it simply changes your shift points, no overall effect on performance.


Yes it would…higher revs for a 3.54 than a 2.88…


I think you‘d probably want internal brake discs and then you should only be worried about possible brake line metrification… but I don’t know enough. Just one thing, you know the ratios of your transmission, I recommend that you calculate the rpm you get in the gears that matter and then decide what you think you like best.

Hi Eric…you basic question is still there…you want a spare complet IRS…I think the XJ6 S1,2,3 are all the same size cage and suspension. …but note some IRS units are different dimensions eg E type is narrower than an XJ6…I think there are 3 different sizes but dont have details…i would suggest mearureing yours for comparison at the junk yard…also note that the diff mount brackets need looking at…again E type brackets set the diff at a different angle to xj6…so note yours…Steve

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Yes, all series XJ and XJ-S, have the same gage and suspension, and you are correct that the E-type is narrower.
The cage changed with the XJ40.