I have a 1990 F250 4wd with the automatic with the overdrive and the 5.8 engine. What kind of gas milage should I expect on thwe highway if it has a decent tune up? Thinking of making it my daily driver for work.

 

14-15mpg or so depending on gearing and how you drive it.

 

The best my '92 F350 4x4 w.5,8L/E4OD ever gets is 12 MPGs running 65 MPH empty on the interstate. I have 31" tires and 3.55 gears.

 

In this case, its your tires that are taking away on what you could be getting.

 

The tires are factory height, but a little wider. But I also agree the factory 3.55 gearing is not helping at all with my combination. It looks good on paper but in practice it is not optimal.

 

I have a 97 F250HD 4x4, 5.8L with an auto and 4.10 gears, it's all stock except for a 3in exhaust from the manifolds and I get about 12mpgs. That is driving at 65mph and I am a fanatic about regular maintenance so it's tuned up pretty well.