You need to figure out the rpm at 65 to 70 mph, if it was pretty low, say 2000 or less, then 15 would be about right. Your stock engine makes very little torque at low rpm so you are required to step harder on the peddle to keep the speed up. I would bet the engine likes to run about 2300 rpm, and if the wind isn't to strong, 16-18 shouldn't be out of the question.
What is the rear gear ratio?
What about tire size?
Just don't expect better milage because the rpm is lower. New cars and trucks are designed for low rpm, yours wasn't.
Joe
