Hello everyone. Looks like a great site for old GM's.
I have a 1988 G20 conversion van and I am getting horrible fuel milage.
The motor is a 350 TBI, trans is a turbo 400, rear gear ratio is 3;08.
I am getting 10-11 MPG. Would switching the rear gears to a different ratio help. OR should I start looking for a 700R4 trans.
Thanks for your help with this issue.

 

That 3:08 sounds real high for a Van. I'd go with a higher ratio to start with. Something like a 3:73 seems more appropriate. I can't assure you that it would improve your mileage, but, it certainly would unload the engine, and cause you to apply less throttle pressure to operate at a given speed. That can equate to better mileage.