I am currently running a 6 litre/4L80E combo in my 1989 XJRS (original 2.88 diff) and have been getting a proven 10.3 litres per 100 km.
After redoing exhaust recently in 2 1/4" and eliminating main silencers/mufflers my consumption has gone to 9.6 per 100k.
This makes the V12 the most economical vehicle l own, who would have thought it possible. All this driving is legal speed stuff of course and l am under no illusion as to what it could be if l put the right foot into it with serious intent.

 

Wow..Really ? That's over 22 US MPG or 27 1/2 UK MPG !! My V12 gets about half that...

 

No city driving in that and here in the speed camera capital of the world (state of Victoria, Australia. ) sitting on 100kph @ 1800rpm no more than a fast idle. That is a proven fill up calculation. I am also very impressed with the accuracy of the trip computer which gives a very similar reading.

 

I can't wait till I get my 4L60 installed, With 3.54 and the TH400 (3000rpm @ 100km/h) I get 16L/100km

 

How hard is the 4L60 swap? Is it a possible DIY project?

Thanks

 

he did a 6 litre/4L80E combo keeping his 2.88:1 rear...


I think some one did that using the 6 liter from a newer xj12 with no distrib. and had to use an after market computer...


using a 6 liter and a 4L80E from a 94 -> 96 xjs might be a tad easier...


up your fuel pressure after doing the calculations... (cheaper than new injectors...)

 

I just picked mine up with the 4 spd auto and 3:54 rear end. It is a perfect combination. I got 18.5mpg on the way home turning about 2500 rpm at 70 mph. Shift points with that combo are dead on. Much more fun car - faster on the take-off and better cruising. Perfect combo - that's what Jag should have produced. Go for it!

 

I have not installed the 4L60 yet, but I believe its a pretty simple swap. You will need
an adapter to mate the trans to the V12 block
spacers for the torque converter
4L60 plus torque converter
shortened drive shaft
rear trans mount

Kickdown cable and brackets if pre-4L60E
or TCU and wiring if 4L60E