First things first I understand it is a huge truck with a big engine. I do not expect great millage... 08 Megacab 4x4, 5.7. 34k miles

But since I have had it the millage has dropped from 16 on the highway to about 11. I have went up to a 285 tire but I have a superchips and have reprogrammed to the correct size. GPS and speddo are dead on. Oil and filters changed regularly. no engine codes.


Any ideas?

Greg

 

Tire size makes a big deal. I went from 235/65/17s on my Grand Cherokee to 265/65/17s and lost a solid 3-4 mpg. You wouldn't think just a 1.5" increase in tire size would have nailed me that much, but it did.
Tread pattern is a major contributor also as a highway tread offers less grip and resistance than an A/T and a lot less than an M/T tire. In my case I went from a soccer mom, passenger car tread to a true A/T tire...

The fact that you are now using winter blend gas ain't helping you either...

 

Did the drop coincide with your tire change or was it starting to get worse before the change?

 

Is it pulling left/right? Noticing the brake rotors getting hot/ turning blue?

 

The drop happened long before the tires. Rotors are normal. It had started before i got the superchip programer also. I have tried running no tune, milage tune, 91, and 87 tunes.

Just strange that it has dropped this bad. On my trip to ky it got a high of 13.2 on the computer and a hand checked avg of 12.8.

 

My 08' Did the same thing, I figured i was just driving too much untill I found out the MPG, it went from 18mpg (about 50/50 highway/city) to 12mpg.

 

Maybe because of winter blend gasoline? My MPG also dropped without doing any changes in my truck, winter tires + winter gas = drop of 2.5 MPG

 

I'd reset the PCM and see if that helps. I wonder if the reset would help with the "Winter blend" fuel change over all?