Big Decrease in MPG

[ross1147]

For some reason my '03 has been getting horrible fuel mileage. Until last month I was getting 20-21 mpg going 70 on the freeway. Last month I moved to Oklahoma with a load of 1300 lbs in the truck. I averaged 15.5 for the trip. There were no hills and little stop and go. I did manage to run the thing out of diesel because the light never came on. When I filled it back up, (I was close enough to station that I coasted in), I realized I still had 4 gal left in the tank. Took it to a dealer and they said that that happens some times. Just the way the fuel is sloshing around in the tank I sucked some air in. Sounds weird, but I guess they know what they are talking about.

Anyway, since the move I have fueled up 3 times with an average of 15.5 mpg. This has all been freeway driving with no load. The last 2 tanks I have kept the engine under 2000 rpm and only traveled 60 mph on the freeway and I still am getting 15.5. I have used injector cleaner, tires are at the proper pressure, and it has a new air filter in it. Does anyone have any ideas on what is going on? It is always windy here, like 30-40 mph winds, will that affect it that much?

Thanks

[Mark Rinker]

At what interval are you changing fuel filters? 10-15K max miles.
Check for any restriction in the exhaust.
Check for low boost problem - loose fittings, wastegate stuck open, etc.
What fuel are you getting in Oklahoma? Maybe you are seeing a regional supplier difference in quality, bio-diesel mandate, etc...
I have run 1qt ATF through 1 full tank of fuel until light exhaust smoke problems subside. You might have one or more injectors starting to go, sometimes this can correct that problem before "full on injector problems" occur (massive white smoke at idle, oil dillution with diesel, fuel knock, etc.).NOTE: My methods/opinions/observations relative to #5 are not endorsed by GM, TDP, etc. and therefore use this idea at your own risk. However I have done this many times with only good results. Have your injector balance rates checked by a professional if none of these ideas work to correct your low mileage problem.