Improving economy

While working for an NGO in the medical field in S.E. Asia, I was given a 4 wheel drive vehicle that suited the area. It was formerly a 1961 Mitsubishi jeep built for the U.S. Air Force. It had a small industrial diesel engine fitted into it some time in the distant past.

My problem. The gearing for the vehicle was originally for a gas engine that turned 3600 rpms. The diesel engine goes red line at 2700 rpms. When I drive on the highways, I get 40+ miles per gallon provided I drive below 40 miles an hour. At 55 miles an hour I get 15 miles per gallon.

I am running the largest tires that can fit on the critter to improve road speed a little. Any thoughts on how I can improve my economy more?

Any room under there for an overdrive unit ?
Gear Vendors sells add on overdrives for many applications.
Not cheap though at about $3000.00

Or just replace the transmission with another with more gears. If you can find one that fits.

Sounds like a neat car. The cheapest may be to change the rear end gear ratio. Hope you have enough gears in the transmission to otherwise to compensate. Best left to a professional and only if you can find the parts.

The original transmission was replaced with a Mitsubishi improvement over the original jeep. This gave it 4 gears and the ratio is slightly overdrive. Unfortunately, this is the smallest jeep made. After the diesel chewed up about 10 drive shafts a creative mechanic built a drive shaft, sans shaft, to fit in the 14 inches between the back of the transfer case and the rear end. I know that, if I could install and overdrive and run the engine down at 1500 rpms my economy would increase drastically.

As for replacing the rear end, finding a matched set of higher speed differentials isn’t likely to happen in Thailand where we roam.

I was hoping that someone has already addressed this shortcoming in jeeps and created an aftermarket transfer case that has overdrive. (I’m dreaming, right?)