I get about 17 mixed and 10 towing the trailer with a truck on it. I stay around 10. it doesnt matter if im towing empty or loaded. I usually go 5 - 10 mph over the speed limit.
You don't to spend $700, you could send your ECM to somebody to tune it for alot less. Or just have your truck tuned by a local. My buddies said Gene tuned there truck for about 250 only..
Check out my sig, I have acouple tuners for sale too.
Using EFI with a dsp5.... Made the trip to san diego a week ago and avg'd 16.8 running 90-100mph the whole way. trip back it was up around 18.5 running a tow tune at ~80.
I've gotten up to 22 mpg hand calculated on a PPE stage 5 tune driving 70-75mph on highway miles only. I am roling 35" tires on 20" rims with 4:56 gears. I also have the Suncoast 6 speed module added.