Dan,
My century has the 14L 470 detroit with the autoshift. At 75 mph we are turning 1550 rpm. we still get only around 7mph. running the twin screw effects it alot as well as turning that many rpms.
Wick

That sounds about right. At 74mph I'm right at the edge of the green arc, about 1600rpm @ 6.8-7.3 mpg. I don't have as aerodynamic fairing over the cab as you do though.

I can't get those numbers either. I experimented the last time that I had the rig out. I finally got 10 mpg, but that's without the generator and keeping it in an rpm range just slightly above 12th gear kicks in. I don't recall the rpm's, but I was doing a little under 65 on flat ground and a reasonably full normal load. I was also falling asleep at the wheel at that speed.

.....68 mph = 1450 rpms.....gets anywhere from 8-10 mpg.....any faster and I get tickets......really don't care about the fuel mileage.....I spend less than a grand for all fuel- lawnmower, trucks, car and house in winter...at this time thats total about 8% of my income a month so it hasn't bit me in the shorts ....yet!......geofkaye