I have a 2015 Chevy 2500 with the 6.0 gas motor and when I bought it early last year it did good for fuel mileage but here in the last few months it’s been getting worse. I’ve changed the oil and filters the same as I always have changed air filter when needed. Nothing is different and it went down bout 2mpg. Dealer cheacked it out and nothing is wrong. I have a good relationship with the dealer so I trust them. The service manager told me to go check out some tuning and do a intake and exhaust and it’ll help a lot. Has anyone went that route? Any good timers that could be recommended would be a great start

I find that buying fuel at name brand locations seem to make a difference for me. I have bought at an off the wall location before for need and could tell the difference in the mileage. Of course it all probably comes from the same place.

When I had my 2004 2500HD with the 6.0 the best I saw was 14, that was 1 trip. Every other tank was 12.5.

It could also be the weather and fuel mixtures. All my diesels get better MPG’s during the summer due to higher outdoor temps and no winte blend fuels.

It should be the opposite for gas motors, better fuel economy during the winter, but that’s just my observation.

I typically only get gas at 2 stations so the fuel it’s self shouldn’t be the issue but the warmer temps I guess could be part of it. I’ve only had diesels for the last 7 years and I got tired of the maintenance and not “needing” a diesel so I went to a gas rig but when I first bought it I was seeing 14 mpg and got 17 mpg on a trip to Louisiana but now I’m seeing 12 mpg and I drive 90 miles a day of pretty open highways without traffic. Guess I may look at a tuner or something cause that 2mpg is going to add up once the summer hits and we start traveling. Wish I could talk myself into buying a gas saver but I hate little cars

Have you recently added a bumper, steps, tires, headache rack etc? Does it sound like the cooling fan is running all the time? I’ve had to replace a few GM fan clutches and they will hurt your fuel mileage.

How many miles on it? May clean the MAF.

Very good point redchevy. I went up one size in tires a few years back and it made a difference.

I’ve got 42,xxx miles on it and it’s still got the same set of Michelin Tires on it that came on it and I added a frontier front replacement bout 6 months. I don’t really notice the fans kicking on anymore than normal

Add on bumpers etc. will affect your fuel mileage.

I would clean the MAF and throttle body and see if it improves.

Sudden drop in MPG or gradual? I'd dump a can of Seafoam per 7 gallons of gas in your tank and run it through. Maybe change the fuel filter after, see if that helps. They don't call it tuneup in a can for nothing... New plugs may help too but it seems early. Start with easy things before you try tuning or exhaust, unless you just want to go that route. If it was getting good mileage with the stock setup should be able to get it back without the extra mods.

I wouldn't do an intake or exhaust mod, you will never get the return on it.

You would be surprised how much a replacement bumper will hurt fuel mileage. I pulled a small trim piece off under my front bumper and I'm pretty sure that knocked my fuel mileage down 1 or 2 MPG. Just remember that $8,000 you saved buying a gasser. That will buy a lot of fuel. And your engine will be more reliable than almost every diesel on the road.

I run the same setup when I notice a drop I will add some Techron,it’s a chevron product! I have some customers that run it in every tank!

Just had a thought. Are you using your AC now?

You can disconnect your battery overnight to reset the computer and it may return to old fuel settings.

Are you actually checking your mileage or going by the computer on the vehicle? I’ve actually never checked mine manually but when Rattlesnkake Dan posted I’m thinking I need to really check mine.

I calculate every fill up that’s how I noticed it. It’s been more of a gradual thing then all at once. The front bumper was put on awhile before I noticed the drop in fuel mileage. I’ve ran some Lucas through it but not seafoam or Techron. Idk just kinda weird it started doing this. I typically don’t run my a/c unless the wife and son are with me I just go with the window down year round except when it’s stupid cold. I’m not really wanting to do the mods cause it runs good I wouldn’t mind a little extra power when I haul stuff but I can live with the way it runs now. I’ll try the seafoam or Techron and see if that helps. Just thought I’d ask if anyone else saw the same thing I know these 6.0 trucks are very common and good trucks so figured there would be some insight on what could be the issue.

Hasn't been mentioned yet, but check the pressure in your tires. Slightly deflated tires can impact your mileage as well.