Gas. Dead stock with 13,000 miles on it at this time and I've not changed anything but the oil and filter (M1 synthetic). I don't want to do anything that's going to cause me detonation issues when pulling 10k lbs up an 8% grade for 8 miles (I see that when I travel east like I did recently with that enclosed trailer). I need this truck to pull 10k lbs, but I could use a bit more mileage (and extra power is a nice bonus).

 

my 04 silverado with ls1, cam(219/228 560 113lsa) , 4x4, 32" tires, e-fans, 3.73's extcab, shiftkit, rolling at 85mph got 21mpg. this was from maine to arkansas. 6 people in the cab and the bed was full of boxes for christmas and clothes.

04 tahoe AWD efans and tune 13mpg.