i've got an 04 z71 ext cab with a 5.3 with 80,000. my prev. truck was a 99 z71 with a 5.3 with 275,000. now i know that they're pretty much mechanicaly the same truck, but i have a issue with the gas mileage with the newer one. everything on the truck is stock except for the tires. they are 34/10.50-16lt. swampers. i bought them new with the other truck, it got 15 mpg. the problem that i have with this 04 is that it only gets 10 mpg. i really need some help.

 

rear-end gearing? Hwy or city mileage?

 

diff 3.73, 4l60e trans, not sure what the t case is but that shouldn't matter. the only difference with my old truck and my new truck is the old one was cable controled and this one is electronic. HATE IT, most of it's highway mileage.

 

pcm re-calibrated for the tire size?

 

Something definitely isn't right. Even I get better mileage!

 

i've checked everything i can think of. air filter, maf, tps, o2s, the only thing i can think of is an evap code for a perge valve. is there anyway that the ecu can go into like a default mode??