There is a point of dimishing returns to that idea.
The ideal cruising speed for an engine is that speed at which it makes peak torque. This is true at whatever speed (and guaranteed that you will be cruising at 5252RPM or less...)
Peak torque RPM is the crankshaft speed at which the engine is experiencing peak volumetric efficiency, and a relatively low Brake Specific Fuel Consumption (fuel used, in pounds, per horsepower generated - I'd have to look up the unit time, but I think it's per hour.)
If you try to work "away" from the torque peak, you will find that your fule consumption will actually go up slightly due to the decreased efficiency of the engine.
CASE IN POINT - My DD is an 88/4.0/BA-10/3.07. Using fifth gear, I was getting about 18-19 highway MPG. I did a little math, and found that I was cruising on the highway about 1500RPM lower than torque peak. A little more showed that in FOURTH gear, I would cruise at about 2800 RPM, in FIFTH gear I was down around 1600 (which I already knew.)
I stopped using FIFTH gear, and saw an increase in fuel mileage from the 18-19 I mentioned earlier to about 22-23 highway when cruising at about 2800 RPM, which illustrates my point.
This is something I keep telling people when they are trying to select gearing - you have to account for your cruising speeds (which I typically figure around 35/45/65MPH) for daily drivers, as well as tyre size and transmission ratios, before you select axle gearing. I'd have to check my notes, but right now my OD gear is useless until about 90-95MPH (if I want to retain cruise mileage) - or until I put in 4.56 at the axle to complement my 31's
Try it sometime and you'll see similar results - "ye canna change the laws of physics!" I'd be happy to help with the math if you need it, but it's really quite simple (I'd just have to find the equations again...)
For a six-speed gearbox, I'd be more interested in DEEP primary gears (perhaps GRANNY-2 and SUPER-GRANNY-1) than OD and DOUBLE OD. Put 1st down around 7.5:1, 2d around 4:1 (and space the rest accordingly) and I'd probably take about a 28-25% overdrive with matching axle gearing for a good box. With that gearing, I'd also take a flywheel with an inertia ring or loaded with Muntz metal for low-rpm, high-load takeoff. What do you think?
5-90
will be the first to attempt a swap into an XJ? Any takers? Is it even possible (seems to me it is). I know it would be expensive but would it be worth it for what would be gained? I would like to be the first but I dont have the $ to try it at the moment. But if anyone wants to be the first I will gladly load up my tools and lend a helping hand!
