88 F250 question

Does anyone know the best efficiency RPM for an 88 F250 4X4 with a 351W and a C6? Other than zero, I mean.

700 at idle? Your C-6 has no overdrive so at 70mph you should be running at 3500rpms, 60mph 3000. That's what it was with my 88 Bronco with a C-6 and a 351W, it was part of the reason I got 10mpg. Hop your doin better.

Well , I was getting 12-13 mpg highway until I put a medium camper on it. That air brake has brought the mileage down to 8-9 everywhere. I try to keep the RPM's below 2500. Seems the mileage goes down really sharp above that. 2500 will get me 57 mph, flat road. Just wondered if anyone knew some specifics about this engine. Some engines have a sweet spot in their RPM range that seems to get much better mileage. I don't mind going slow, I just want to be able to buy a hamburger after getting gas.