I've towed out 27T with both a '99 F250 7.3 diesel, and now with our 2003 7.3 diesel Excursion. Both have/had DP Tuner chips, with the 60hp tow, and 80 hp Economy settings. We live at 4500', and have to negotiate many mountain passes on our trips, so factor that in. Towing at 65 mph we consistently get 14-15 mpg. For example, on the last trip the first tank yielded 14.8 mpg, hand calculated. Two years before on a 2000 mile trip, 14.7.
The max I ever have gotten with the '99, set up with a Gear Vendor gear splitter transmission, at 60 mph, careful driving, truck unloaded not towing, was 22 mpg. That's once, and that is MAX. At 60 mph my normal average, well documented for business reasons, was 19.x. At 70 mph highway cruise average was 17.X mpg. And I drove like a crusty granny to get that mpg.
DO NOT believe the guys that post consistent average mileages over 20 at tow speeds ! No-No-No
: They use a vivid imagination, and go by the 'lie-O-meter' mpg gauge in the truck, which is never accurate. There is a whole lot of BS going around on the diesel forums, and based on my personal experience and those of people I know to be honest who own diesels, I know you will get mid to high 'teens. No magic bullet. Best mileage modification is a numb right foot.
However, nice to have that diesel power!
Good towing
Reno
2003 Excursion, F250 X/B springs, sway bar, HD steering stabilizer, Two Aux tranny coolers, trans in-line filter, DP Tunes, Edge CTS Gauges
Gone but not forgotten: early '99 F250, 4" exhaust, DP Tunes, Gear Vendor aux trans, aux trans cooler, X/B springs, aux gauges