What is the gas mileage on 1990 7.4 with granny transmission?

Typically if this is a Chevy 1/2 or 3/4 ton two wheel drive with 373 or better rear gears you should get 10-12 mpg. If this has dual wheels and 410 or higher you might get 7-9 possibly 10 if you got the wind to your back and are on the decline of a really steep hill.