1990 G30 Gas Mileage

Can the rear end differential on my 1990 G30 Sportvan be changed to increase gas mileage? The engine runs high RPM'S @ 55 MPH. The van has a 3 speed Auto Tranny.

If you don't haul heavy loads and usually just hit the highway then you should be able to change ratios, but it isn't trivial. The speedometer won't be correct unless you have it recalibrated. One thing you might look into are bolt-on overdrive units that are designed for RV's. They usually bolt onto the back of the tranny and will give you overdrive. They aren't cheap, though, and the driveline would have to be modified and rebalanced.
What transmisison do you have? I'm surprised it isn't a 4-speed OD unit (e.g. 700R4 or 4L60 or 4L80).

Richard

If you don't haul heavy loads and usually just hit the highway then you should be able to change ratios, but it isn't trivial. The speedometer won't be correct unless you have it recalibrated. One thing you might look into are bolt-on overdrive units that are designed for RV's. They usually bolt onto the back of the tranny and will give you overdrive. They aren't cheap, though, and the driveline would have to be modified and rebalanced.
What transmisison do you have? I'm surprised it isn't a 4-speed OD unit (e.g. 700R4 or 4L60 or 4L80).

Richard
The transmission appears to be a THM400 from on-line depictions of the bottom cover plate. It is definately a 3 speed, no OD on the steering column indication.

SANMARGIL

I don't know if this is practical or not, but if you can find them try some larger wheel/tires (e.g. if you have 17" rims try to find 18" rims, or see if you can get larger diameter tires). This would give you the same effect as a rear-end change, and you could decide if it was the right direction to go. But there's nothing like an OD transmission that locks the torque converter.

insted of changing out your whole rear end try a ring and pinon... 5 hour install w/ experience 400-500 bucks