2008 Chevy Silverado 6.6L Duramax fuel problem

Do you have the factory size tires on it? Add a quart of transmission fluid to the fuel tank to clean the injectors. Air filter new?

My driver door says it should have 16 inch tires, but it has 265/17's on it and I'm pretty sure it came with the 17's on it. It's funny you mentioned the air filter. I had the tech check the truck out last night, and through conversation, we pulled the air filter out and it was grey and you couldn't hardly see light through it. I bought a brand new one today, but haven't had enough time to see if it helped.

That is a big part of it. You can also buy a chip for it and set it in safe tow mode and it will increase the mileage and power. Usually 3-7mpg gain. I run superchip in mine.

Also, are you referring to Dexron III transmission fluid? I have heard of adding transmission fluid to the fuel tank, but wasn't sure it would hurt the engine

I pull a 33ft fifth wheel camper with my truck or I will be. I just purchased the truck and figured I would be getting a lot better gas mileage. I have not pulled the camper yet to see what mileage I get with it behind the truck. I sure hope the air filter helps

Yes that is fine. One quart to a full tank of fuel is good for it occasionally.

Ok.....I'll try it and see if it helps. I appreciate your help!!

no problem.

tans fluid in the fuel tank is not recommended. Have replace many injectors on duramax's for people using "old school" tricks.

Marvel mystery oil works wonders in fuel gas are diesel it cleans your system out an saves on gas one quart per tank don't matter how many gallons

I think you should look at tire charts and see if your tires are different height from the ground to the top of the tire. 16 and 17 inch is just the rim size. The 265 is the more important part of the equation. That is the amount of rubber from one side of the rim, up and across the tread and down to the other side. The 65 (the second number) is also important as this is the ratio of side wall to tread. The bigger the number, the taller the sidewall. Next you should see what size tire is stored in your computer's programing. If you like the tires and rims you have, get your mechanic to change it. The simplest way to check if you really have a mechanical issue or a tire size issue, get the Speedometer app from your play store. Start the app and prop your phone on the steering column next to the speedometer. See if they match. I was slightly surprised to find that my 2015 2500HD with 6.6 ltr Duramax was getting 20 mpg on the highway. My tires are 3/4" taller than the factory tires and runs 58 mph when the speedometer says 55 mph so... Good Luck!