GM transmission options?

I’d like to put a cruising transmission in my xj6?? When I’m doing 80 the races are around 3500 rpm. What transmission is better for crusing?? Will I need adapter plate and will I have to figure out gear changer?? And driveline and mounts, Speedo etc. Please anyone give me some options so I can start getting a transmission and also what American car and year will have…

Thanks for any suggestions

Rpms/road speed is a combination of gearbox and diff ratio, Sawyer - and most automatic boxes has a 1:1 in ratio. Unless fitted with overdrive; swapping gear boxes is not a solution, but the manual box have 0,883 in 5th. Unless you swap to a manual box with a higher ratio in 5th gear with a 0,883:1. You may also consider swapping the diff, if relevant - you’re not always cruising at 80 mph…:slight_smile:

However; at high rpms the engine develops plenty of power. But as most countries have speed limits around 60 mph Jaguar ‘geared’ the car for such lowers speeds. At lower speeds and a high gear ratio; the engine rpms will, at one stage, develop too little power for comfortable cruising in varying terrain - hence the use of selectable overdrive. Even a slight incline, or strong headwind, may overtax the engine with a high gear ratio at lower speeds - forcing unwanted downshifts. Hence the overdrive, which can be switched on/off at will - as an option for the standard Jaguar set-up.

These inherited power and torque curves of the xk engine is a constant factor; it needs rpms for power, and can stand it…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

In other words, it needs more gears? All the e-type and manually driven tacho XJ S3 conversions should work on the series one I‘d say. Consider a 3.056 differential or close.

no affiliation, is USA based, been around for ages


That would be all-round best solution.

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Hi Sawyer. As @awg said, the John’s cars GM 700R4 conversion is a great option. It’s an overdrive transmission so it’s just what you’d need for highway cruising. Also plentiful in the US cause tons of GM cars used them. There are a good many XJ’s running around with that conversion so alot of support. Can’t tell you offhand which GM cars and years exactly but that’s an easy google search or check the John’s cars site.



Frank thanks for explaining drive train theory! I take my jag quite often from California to Montana 4 times per year and I do a lot of freeway driving. Maybe the overdrive unit I need to search for then . Or change my engine.

Where would I find a 3.056 final drive? Thanks david.

@Dick_Maury may have an answer for you.


with my UK spec car I have to agree - 70 mph at +3000 RPM is way too much. The SII was delivered with a 3.31 (43/13) ratio to the UK and other markets, but with 3.07 (43/14) to i.a. Germany as autobahn driving (not cruising) required a longer ratio. Some of these diffs were even Powr-Loks. If you keep watching out you may run across one of them. If original they carry metal tags with the ratio and “BP-L” for the LSD.

A 4.2l engine in good nick - and with twin SU carbs or FI - should easily pull the longer ratio. The big difference is probably city and suburban traffic. The short rear axle will be quickly in 3rd and most of the time just remain there. The long rear axle might lead to some untowardly up- and downshifting in suburban traffic, but I never heard of any complaints even for the long ratio German models.

Good luck


75 XJ6L 4.2 auto (UK spec)

My S1 has a 3.07 diff. It’s all a matter of pedal pressure and vacuum (BW12). It quickly moves to 3rd as they are designed to do and kicks down as necessary. I don’t race at street lights and the lower cruising RPMs are appreciated. There is a 2.88 option as well - might be a bit tall for my liking. Paul.

The GM TH700 is a good option as it has a 4 gear that is overdrive. I would leave the 3.54 in the car if going with this conversion as the engine is not strong enough to pull overdrive in a 3.07 or 2.88. The XJ6/3 used a 2.88 final drive ratio without overdrive so this might be a good option. Readily available easy to find diffs. There are 3 different versions of the 700R4 transmission so do your homework. Plastic gears are available for changing the speedometer to match your choice of rear ratio. This conversion does use a solid rear mount which is kind of noisy compared to stock. I ended up modifying to use a spring perch with the GM transmission on my XJ6/3. As with anything not designed for the car, there are a lot of compromises. You did not say which year your XJ6 was but from the revs, I would guess a XJ6/1. If going to a 2.88 rear, get a torque converter out of a early V12 XJ or E-type that use the BW12 as it will have a 1800 stall speed vs 1200 for stock. This will get you off the line a lot better to make up for the ratio change. Jaguar did this on the XJ6/3 and makes for a very driveable car.

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Craig thanks for info I have a dana final drive in my 72 xj6 it’s from a 83 xj12 so I’m going to look at the tag I have the BW 12 granny with 70 tires. But my Speedo is of 10 mph. So I need to work out that I have 75 size tires on my 86 xj6 but that car don’t tec has high at 80 rpm are around 3200 rpms.

Paul I’m going to look today on my tag at final drive I’ll report back later thanks.

Dick I have a 72 with xj12 final drive but will check today I do have xj12 torque converter also. I do have xj6 rear from a 83.

I once saw a reference to the 700R4 as a “power sucker?” I.e., it was inefficient.

Didn’t seem to ring true to me…is it?

You could try this 2.88… you have the same power as that car had and yours is lighter so it will work. Maybe the low stall speed (does that increase efficiency?) hurts off-the-line and thus you should go for a higher stall converter.

The 700R4 is designed for a American V8 with lots of low end torque. Put it behind a engine that puts out power at higher revs and it won’t be as happy without some modifications. On my own car, I selected a 2000 RPM torque converter and changed the rear diff ratio to a 3.54 posi. Car was faster off the line and with the overdrive 4th, had the equivalent of a 2.60 diff ratio. Better acceleration and lower RPM cruising. Kind of a win-win. I did have to play with the governor weights and modify the bell crank for the TV cable. The Jaguar pulled it out to far so had to shorten the overall throw. If I remember correctly, it is supposed to be 1.1" total. The stock FI XJ6 was much more.

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I have two final drives from xj6 both tags say 2.88. ??so any good? I go to salvage yard! There was a xj12 there 1979 with gmc stamped on the trans pan so any one know what type of trans this would be but don’t look like same bolt pattern has xj6 can so!some educate me thanks.