Bad fuel economy help!!

[More Power]

took my truck into the dealership today for piss poor fuel economy. They said that they could not find anything wrong with it. I thought the truck was stuck in regen, but they said not. They were on the phone with tech support with no help there either. I told them that it went for 16 to 17mph in town to 10 to 11. and when they drove it they said that they were getting 11mpg , and that is normal for a 08. I told them that it bull shit and that there is no way that I'm only getting 150 miles to160 on a tank of fuel on flat ground on the way to and for work.They also stated that it could be from changing the rim and tire size to305/55/R20.No way!!? since the fuel filter is just changed and oil. They don't know and have to call tech. support in the morning. I'm out of ideas Can anyone HELP!!!??

[MRBASS]

Did you have the 20's when you got 16-17mpg?
Tires can be a killer. I went from stock 265-17's to 285-17's and lost 1-1.5 mpg. Hard to believe I know.

[More Power]

Not for sure????? I guess I could only hope not. It Happpened on a 600 mile trip home. is when i noticed it the most..and I did have the larger tires on at that time.

[mycirus]

You do know with the larger tire your speedometer and odometer will be off, right? My guess is nothing is wrong.

[More Power]

I'm pretty well spot on with the MPH Thanks.

[Dmax 5th Wheel]

Tires with aggressive tread & that size tire/wheel will make your MPG go down. &/or
you got a bad batch of fuel from that trip.

[GDD]

Originally Posted by More Power

Not for sure????? I guess I could only hope not. It Happpened on a 600 mile trip home. is when i noticed it the most..and I did have the larger tires on at that time.

The mileage on my 08 was never that low, even when new. I'm guessing that you went from stock rim/tire size to the 20's. Not sure if I understand if you already had the 20's, were getting your normal mileage & then the mileage dropped suddenly on the long trip.

Probably doesn't matter, but is the low mileage reading from the DIC or hand calculation? Did the mileage change at all after getting a new batch of fuel & more driving?

Please let us know the outcome with the dealer.

[Adrexco]

My 07.5 LMM always got about 16mpg average. About 6 months ago it suddenly droped to between 9 - 10mpg. Nothing changed with my driving pattern or any mods done. The dealer finds nothing wrong with it (no codes). I also seem to think it's going into regen more often. I use to be able to get 22mpg on long runs if I kept it just under 65mph. Now I'm hard pressed to get 17mpg driving it like a granny. Something changed, anyone else have a similar experience?

[mrgransport]

I am experiencing the samething on my 2007 new body style. Up until 2 weeks ago I was getting 15.5-16 around town and 22-23 highway. I am now am getting 10.6 around town and 12.8 highway. Also having problems with diesel exhaust filter. It had to be regenerated twice in one week by using the method in the manual. Also my oil life went from 90% to 78% in 13.1 miles. HELP!!! Chevy says they cannot find anything wrong. No after market changes at all other then a K&N fillter put in 2 years ago

[65turboman]

Do a dpf delete. Wont be a huge improvement, but much more consistent mileage. If your dpf is plugging it will make a bigger difference.