I have a 03 GMC 3500 ext cab 4x4 with the Duramax and the Allison trans. My problem is low fuel mileage. I got 17 to 18 mpg when truck was new but now only 13 mpg. This is highway driving. The truck has 25,000 miles now. Fuel filter has been changed every 5,000 miles and the airfilter every 10,000 miles. I have also installed a Racor pre filter. Everything else on the truck is fine. Any ideas? The dealer has it now. Thanks.

Do you have stock tires? Many people change the tire size and don't recalculate the mileage for the new tire size.


If this is the case you have aprox 4% error with 265's and 8% error with 285's. To calculate the mileage add either 4 or 8% to the total miles driven, then divide by the amount of fuel it took to fill it.


Ex.
odo shows 300 miles since last fillup


300 X 1.04 or 1.08(depaending on tire size) = 324 actual miles (used the 8% error)


Then divide that number by the number of gallons it takes to fill


Result is MPG corrected for tire size.


Bill

Do you have stock tires? Many people change the tire size and don't recalculate the mileage for the new tire size.


If this is the case you have aprox 4% error with 265's and 8% error with 285's. To calculate the mileage add either 4 or 8% to the total miles driven, then divide by the amount of fuel it took to fill it.


Ex.
odo shows 300 miles since last fillup


300 X 1.04 or 1.08(depaending on tire size) = 324 actual miles (used the 8% error)


Then divide that number by the number of gallons it takes to fill




Result is MPG corrected for tire size.


Bill

He has a problem that is a LOT bigger than 8%

I'd be looking for low tire pressure or something draggin' - given that he's changed fuel and air filters on a reasonable schedule. The other thing is whether the type of driving has changed, e.g. shorter trips, heavier traffic, bigger loads, etc.

I have stock tires and the pressures are good. Got the truck back from dealer, says no problem. The dic shows 19 mpg. GM says I should be happy with 13 mph. 400 mile trip last night shows 12 mpg using the old way of figuring, gallons put into tank divided into miles driven. Will try another dealer, this sucks.

I know of more than a couple of GM 2500HD trucks that have brakes dragging, most often rear passenger, due to very rough castings of calipers. take a short drive and then put your hand on the rear wheels. if one side is hotter than the other, remove the wheel and check the pads. They should be the same thickness on both sides. If not, that usually means that they're dragging.