My friend has a 98 1 ton dually with 4:10 gears he bought it new in 97 the truck is totally stock and he will not mod it. the truck only has 60k on it. when he got it he said he could get 15mpg out of it now its only gettin about 10. (fuel filter is changed frequently) does anybody have any suggestions other than a chip (i have already tried) i thought maybe a bigger tire would help since he is running the factory size which was 215's back then, call me crazy but when i put my 35's on my truck i gained a few on the highway. Please any input would help thanks.

 

It would be a 12 valve then all mechanical mods to help it. I bet the timing sliped on it if its getting that bad of milage.

 

I bet it is the ULSD that has messed up the mileage, I heard a lot of people say theirs gone down with ULSD. I have not noticed much difference myself. When I bought my truck it did 12.5 MPG hand calculated. It was in the winter, so I put some PowerService in the tank and in 2 tanks, the truck was making 18.5 MPG hand calculated. I have no idea why - just the fact.

 

His truck is a 24 valve. i have a buddy who has a 12 valve tho and his sliped but the truck would barely run. we thought it blew up talk about a freaky expierence