Ran two tanks through on my way to tenessee and back today, average gas mileage 14.8 on both tanks manually calculated! This is the WORST I've seen since I bought the truck, with 98% interstate driving! Both tanks were BP fuels, 93 octane, what I try to normally fill up on. I was babying the accelerator on the 2nd tank on the way back home hoping I was too aggressive on the first tank, but I ended up with the same result. Cruise control on most of the time, set around 74-76. Usually with that much interstate driving I've seen 18-19 MPG easy, any ideas to the sudden decrease??? I'm taking her into the dealership tuesday to have the t-stat checked as it seems to be running cold, and my heater is horrible, but other than that I haven't noticed anything else different...

 

Whats your mileage? When did you do the plugs last. My mpg is down, and i feel that is the culpret. A lot of people see a huge difference when they change them out.

I was hoping to push the plug change out to 50k, but looks like i wont be able too.

 

Ummmm... I hope that's not it... Mileage is only 6200, bought the truck 10/05/07

 

why are you running premium? Midgrade (89) is recommended for "optimal" performance.

 

My truck idles rough with mid grade..

 

Well, its not plugs then . You shoud be running midgrade though. too high octane is just as bad for a hemi and too low.
sorry i am of no help to your problem though

 

Your speed and if the highway had a lot of inclines is probably part of the problem...cruise control and inclines don't work good together. Also because of your stat problem your engine is running colder then normal...that's really bad for fuel efficency but good for the actual engine.

 

All of these trucks idle rough, it's not because of the gas.

 

first things first... the fuel has just recently changed to the winter mix..... also driving habits could hurt... unless you have a tuner dont run the high octane....

 

OK, even if he has winter mix fuel it is only E10. So that small change will definately not give him the appx 22% change he is seeing.

But I do agree with what someone else said about the great amount of hills that could cause crappymileage. This could have been emphasized by the E10 winter mix.

What is your tire pressure too?