What is the rpm sweet spot for highway driving

Consider the speed at which you DRIVE. Do you drive 70 or is that just where you calculated? I couldn't imagine driving 70 even on the interstate. Just doesn't make good economic sense. When I had my Valiant, it had 4.30 gears. I drove that car everywhere, all the time. On the interstate with my big toe on the gas, just to keep it cruising at 55, it was turning a tick over 2700, that's ALL.