man, i can't imagine there would be that much diff. i had a very late 05, 06's were on the lot when i bought it. the best i ever got out of it was 16 on the way home to vegas from idaho (650 miles all freeway 80 mph.) after that i avg. 13. i drove it stok for 13000 then put a chip and 4" straight pipe except for a flow thru muffler. it didn't make a diff. my chip was an edge and i know i could have picked a better one but i never saw over 15 after the first trip home. i got 10 pulling a 24 ft hauler. about 7000 lbs id say. it had 36000 on it when i traded er in.
not smoking nothing dude. speed limit here on the highway is 55. run about 60 all the time. i saw a 5mpg gain with the hypertech programer alone, and running taller tires. i do mostly highway. when i do get to driving around town im at 19, eeryone says im full of shi* when i say it, but when they ride with me across four states and do the math they become beleivers. i cant explain it. my grandfathr hs an 07 thats stock and he gets 20 on the highway. my boss has a stock 04 he gets 13 and my broher has an 03 getting 16. im sure there are more guys out there getting great milage that no heres about with a 6.0. i also forgot to mention before that i have an egr delete on the truck
my intent was not to call you out on your milage claims. i have heard other people claim similar numbers. i was more referring to the differences between trucks from the same time period being so different. i don't know when mine was built. it could have sat on the lot for a year for all i know.
you need to get rid of the lift kit and go back to stock tire size and you should then see 17 to 23mpg depending upon the speed stock without a tuner. I consistantly average 22.5 at 55-60 on the interstate but only 14.8 around town loaded or unloaded
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