How do I maximize MPG? from miles per hour

Driving at moderate speeds is about the only thing I can do that has a payoff. The last round trip of 400 miles proved it to me. The route was half on the Interstate (65mph max) and state highways (55mph max). On the Interstate I chose to go with the truck traffic that is restricted to 55mph, but they cheat a little. Many of the trucks do drive at a consistent speed to save fuel and avoid heavy fines. So I just settle in with the pack. On the state roads, I just take my time and try not to impede traffic by letting the jackrabbits use the passing lanes. Normally I get 15+mpg average. On this trip I got nearly 18mpg. Doing a little math, I figure that's a bonus of about 50 extra miles. At 70-75 mph I will get close to 14 mpg. On my last 10,000 mile circuit around the country through 24 states, if I made an effort to keep to not more than 60 mph, I figure I would have saved about $400. And, if you are worried about climate change, the difference would be about a ton and a half less carbon in the atmosphere. I have an idea. If I drive slower, can I get Carbon Credits and sell them through that crook Al Gore?