I checked my mileage numbers today as diesel prices shot up around here this weekend and here is what I got- At 60MPH I get 19mpg. Really great I think, but when I kick it up to 70, (the predominate speed in my area) I dropped to 14.4 .
The tach says at 60-1700rpm, and at 70 about 2100rpm. I believe the rear gears in an 01 automatic F-250 are 3.73. I dont tow at all and the truck is stock except for the Kwik filter. Should I get bigger tires? Change to 3.55 maybe? Get a chip? Looking for some good advice to get mileage up. Almost all my driving is freeway at 70mph+ so if I could get those numbers at 70 I would be in heaven. I know about wind resistance as you go faster but if I could get to 17.5 to 18 MPG that would be great. 14.4 is a little hard on the wallet at $2.30+ a gallon. I run stanadyne consistently as well.

 

I think the gears are probably 3:55, but not sure. You can check by looking at the tag - door jam - drivers side. I run 2100 rpm at 70 and I have 3:55. Keeping the truck below 2000 rpm's is the key.

Axel Code

39 = 3.55 open diff
C9 = 3.55 limited slip
35 = 4.10 open
C5 = 4.10 ltd slip

 

i have a 5 speed 4.10 rear and 33" tires. i get 13.8. a little highway alot of city and some boat pulling.