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I've got two - an 85 with a 6.9 and a 94 with a turbo 7.3. I drove many 7.3 trucks when new back in the 80s and personally never got better than 15 MPG. I hear reports of 20 plus from others though. 6.9 is the same engine as the 7.3 with a slight difference in bore. The gearing and type of trans will make a difference. Overdrive is supposed to make a big difference, but it hasn't made much of a difference with my Ford IH diesels. I take a long trip every year - mostly flat highways with a 2000 lb. pop-up camper on the back of the truck. My 85 with 6.9 (no OD) averaged 13.2 MPG, my 94 7.3 turbo (with OD) averaged 12.4 MPG, and my 92 Dodge (Gertrag OD) with Cummins 5.9 got 17.5 MPG. My 85 F250 4WD ex-cab with 4.10 axles and C6 auto-trans (no OD) and 6.9 gets a best of 13.5 MPH empty at highway speeds and 9-10 MPG pulling a 6000 lb. traier. I took this truck on a 3000 mile trip with a 2000 lb. slide-in camper on the back and averaged 13.2 MPG for the entire trip - mostly at 70 MPH on flat highways.
My 94 F250 4WD ex-cab with 4.10 axles and E40D (lockup overdrive auto) with 7.3 IDI turbo gets a best of 14.5 at highway speeds. Best mileage it ever got was in Michigan flat-land with a wind behind us and we got 17.5 MPG at 75 MPH empty - but that's rare. I took this truck on a 3000 mile trip with a 2000 lb. slide-in camper on the back and averaged 12.4 MPG for the entire trip - mostly at 70 MPH on flat highways.
My neighbor took his on a long flat-highway trip to Florida. From here in New York to Florida is slightly downhill the whole way. It's a 94 F250 4x4 super cab long bed with a IDI TD, E40D, 4.10 gears. The truck was basically empty... one
motorcycle in the bed and pulling a jet ski on a trailer. Cruise Control set at 70mph (about 2250rpm) --> 13mpg Cruise Control set at 60mph (about 2000rpm) --> 16.5mpg Cruise Control set at 55mph (about 1800rpm) --> 21.3mpg The new 2008 diesel trucks were just tested pulling a 9000 lb. trailer. All are four-door cabs and 1 ton trucks. Chevy 6.6 Duramax won at 10.5 MPG. (18.2MPG empty)
Dodge 6.7 Cummins got 8.7 MPG (13 MPG empty)
Ford 6.4 IH got 7.6 MPG. (11.2 MPG empty)
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