I drive a 1990 F-250 Diesel with the 7.3L. Ive had the truck for 4 months now and up until a week ago i was getting 13-14 MPG which i was told was still a lower than normal, but then a week ago I started getting 8-9MPG! Im not driving any differently and ive ran two full tanks through my front tank and one through my rear tank and theyve all been in the 8-9MPG range. Any Ideas on why this may be? This is going to get expensive fast if I cant get this figured out soon! Hope you guys can help me out.

 

Pre-Power Stroke Diesel (7.3L IDI & 6.9L) - Ford Truck Enthusiasts Forums Here is a link to the IDI diesel page, maybe ask the guys there.
13-14MPG is about what I get but my truck has a C6 trans, with an overdrive trans you could expect a little more depending, of course, on how you drive.
You have to be cautious with MPG figures as a lot depends on how you are measuring the amount of fuel. The stock fuel gauges are not very accurate.
Anyway try the IDI diesel page for more replies and good luck.

 

try to provide more details,such as rear axle/trans info.along with location,and the miles on the engine,and when it had it's last tune up-if ever,along with how the truck is used.
people with gas engines are much better about tune ups,than folks with diesels iv noticed.
mostly because a diesel tune up cost hundreds of dollars.but you pay more many times over at the pump,than you would just keeping things properly tuned.

15.01 avg MPG @ 36 tanks.
but im hauling a large wind catching bed,and sporting duals.
keep in mind my avg speed is 45/50ish,very rural,stop and go/heavy hauling,and light towing combo.
you should be able to mop the floor with my average,if your running a stock bed/3.55's/OD trans etc.
also note im non turbo charged,which keeps me a little lower than if i were.

meanwhile,have a read here for some tips:
https://www.ford-trucks.com/forums/9...ing-25mpg.html