I was thinking with a good torquey cam rated from idle or 1,000 to 5,000 rpms, along with good gearing to keep the cruising rpms between 2,000 and 2,600, or wherever the sweet spot rpm is for the engine to make maximum vacuum at cruise, that would allow minimal gas pedal to keep the beast moving along. Whereas a 350 needs more pedal to get the power. I talked to a guy last year that has a C20 Suburban and he said it can get 25 mpg's. Maybe he was pulling my leg. I do know when I ran the 3.54 rear in my other '65 with a stock 350, it consistently got 14-16 doing 70 down the road.
