I get 25 on the open road and 17 in town and 15 towing my boat @ 70mph. And thats with the 35s.
Its all in the mods

And with over $15k in upgrades Ill never get my money back in saved fuel but who cares its just plain FUN.
7.3L with 3.73 LS
Banks Power Pack
Edge Juice W / Attitude
GDS Ball Buster Turbo
BTS Transmission (damn near bullet proof)
ATS performance injectors (Stage 2)
ATS Twin HPOS
Sitting at about 650 HP @ max output cruzing at about 400 HP
For 25 mpg I keep the RPMs at 1800 with is right at 70 mph, and have to keep my foot out of it and resist the urge to race those fucking noisy ass rice burners.