What's the best final drive ratio for highway fuel conomy on a Pre-HE?
 

What's the best final drive ratio for highway fuel economy on a Pre-HE? Has anyone changed differential gears and found the new ratio to be more or less fuel efficient? Overdrive transmissions would also change the final drive ratio, but manuals or lock up converters could artificially aftect the fuel economy relative to ratio.

The taller the ratio the better the hwy fuel economy will be (to a point). A 4speed OD auto with LTC will be doing 1700rpm at 100km/h. But will be slower off the mark than if the diff ratio was lower, so it's a compromise.

Changing ratios requires changing the crown wheel carrier. Going up in ratio say from 2.88 to 4.09 a spacer can be used between the crown wheel and carrier, going the other way will require the replacement of the diff center.

My car had 2.88 and on a 6000km trip across Australia we averaged 14L/100km about 20 imperial miles /gallon.

cheers
Warren

If you have a 3.07 diff then going to the 2.88 won't make that much difference, IMHO. If you have the 3.31 the difference would be more significant...2-3 mpg is my guess.

On my XJR I went from 3.27 to 3.58 and lost exactly 1.0 mpg at highwy speed. I know there are other variables but I threw than in for some sort of perspective and/or real world results.

Cheers
DD

That's the point I'm looking for. I've heard claim that a 3.08 gets better mileage than a 2.88 for some in an HE. With the even higher RPM torque peak of the Pre-HE I wondered if gearing lower than the stock 3.31 would actually lose economy.

crown wheel.... ring gear in American, gotcha. There's no adjustment for different gears? Interesting.

I wouldn't think so as the V12 has a good spread of torque across the rpm range. And Jag went to the 2.88 to improve economy. I have driven one with a 4L60 and 3.31 it went very well.

At 100km/h these are the aprrox revs for the given ratio, not a huge amount so unless someone can chime in with a ratio change it's all guess work.
3.08 2500rpm
2.88 2350rpm

Jag carriers are a bit of an unknown....... But Dana on which they are based have 3 different carriers depending on which gears are installed, so there is some adjustment of the carrier sideways to mate to the pinion.

my opinion is to always go with the highest (numerically lowest) gear ratio possible.
ive got a 4t60e, which is the FWD version of the 4L60e. im making, maybe 20 more horses than an XJS and its a real struggle for grip in first and second. if its wet outside, forget it.

EDIT: im running 2.93 final ratio. i idle at 1700 RPM at 70mph

For some drivers under some conditions, yes, I think that's plausible. In city driving, or moderate acceleration to a given speed...a bit less throttle might be needed with the 3.08 gears. But for highway driving...100-200-300 miles at essentially a constant road speed...I have to think the lower rpm from the 2.88 gears would yield the best fuel economy.

Cheers
DD

IMO, the pre-HE engine is mostly hobbled by its low 8:1 CR and very early computer system which cannot optimally map either ignition events or fuel. It would probably benefit most from an increase in compression and remapped timing, neither of which is a low-cost, bolt-on affair.

Before the HE was developed, there was some work at Jaguar to develop a 10:1 CR version of the V12 engine which had increased power and economy. The idea was shelved for a number of reasons, but largely because the catalysts and computers at the time were unable to produce the required emissions results.

A change in rear gears would likely be less effective than going to an OD transmission. Moving from a 3.07, 3.31, or 3.54 to a 2.88 will cost you a great deal in performance and unless you do mostly highway driving, it could actually cost you fuel economy at low speeds as higher throttle openings are needed to overcome the taller gears at low speed. A 700R4 swap would be a good compromise.

i forgot to mention, although i get nearly 40mpg highway in my V6 grand prix. my in town fuel economy is less than 12.

Aus got the 10:1 engine for the last year of pre-HE production they were the cream of the V12's. I drove one with a 3.31 and 4L60 OD trans. it was very spirited.

I am in the process of a swap at the moment. 4L60e and 4.09 gears, although I may end up with 3.58's I have just bought in place of the 4.09's so I will be able to a direct comparison.

cheers
Warren

Actually the 700R4 and 4.11 was my first thought, matching the Jaguar 2.88 final drive ratio. My primary concern with that setup is that the 12.6 drive ratio in first gear may only be good for producing tire smoke. Second gear would nearly match the original first gear ratio.

warrjon: Looking forward to reports on your swap, and findings with the gear ratios.

Yeah that was my concern as well, I think the 4.09 would be ok with a mild V12 say upto 450hp but as I am planning 500+ I think smoke would be prominent so I bought this for $40.00 yes 40 bucks, an XJ40 rear end you say -WHY- it has a 3.58 LSD center and I feel an outboard disk conversion coming on

Mine will be mild, I'm planning on keeping A/C, power steering etc, so there's little room for power adders.

You would be surprised what you can get in the bay with some careful planning. Lister added 2 centrifugal superchargers with all conveniences kept. Besides a NA V12 can make 400hp from 6.0L.

Here is a link to guy who has done quite a bit of work on his V12 with some good write ups.

XJ-SC Modifications