I don't think you'll be able to maintain 40mpg going up any long, steeply graded hill...going slow works to a point.
If it is constant throttle you seek, why not just hook the throttle servo to a potentiometer.
I'd imagine using a cruise control based on MPG might pose some dangerous situations. A car will shift its speed greatly as it is loaded differently throughout a drive. I can't imagine that driving 65mph, slowing to 40mph, and speeding up again to 60mph would be particularly safe in any form of traffic. If you are completely isolated, then I suppose it might be useful, but not as a daily use item.
I'm interested in watching any developments, though.