[smcguyer]

That is the approach I would use, assuming you have shed as much weight as possible and cannot change the axle.

The simple vacuum guage tells you if you are making changes in the right direction. If you make a change to the vehicle/engine and you see a higher vacuum reading for the same speed, you will most likely be able to get better mileage. Because the throttle plate is closed more thus less air/fuel mass is being consumed for the cruise condition.

Find articles on optimizing cruise fuel economy for that engine. The basics are reducing flow restrictions in the intake and exhaust system (i.e reducing pumping losses). Then increase the charge density of the fuel/air mixture at cruise (i.e. colder air mass allows for less throttle plate opening). Then try to tune/control the engine management to enable lean burn, with increased timing advance... at cruise.

This means you have to override the limitations of the oxygen sensor /cat converter. Some engines respond well to lean burn with advanced timing (at cruise). You push it as far as you can as long as the vacuum guage keeps reading higher vacuum, and you just avoid lean burn detonation. The less your throttle plates are opened at cruise, the better the results you will probably see with this approach.

A Scanguage or other type of electronic guage might tell you the amount of throttle plate opening you have to begin with, and also the vacuum.

On older heavy vehicles with a carburetor, you drove while watching the vacuum guage. This is because you wanted to drive with a high enough vacuum to prevent the carburetor power valve from opening. If the vacuum got below, say 6", the power valve would open and richen the fuel mixture and you got less economy. So you would drive the vehicle in a way to try to realize the highest vacuum guage reading.

You have to find someone who has actually done economy tuning on that engine, to know the results without trying it yourself. Call the engine tuners for that engine and see if any of them have done it (for cruise conditions)....although most changes made to an engine to develop more power also translate to better fuel economy, for all the physics reasons mentioned above. David Vizard is a tuner that has experimented with lean burn alot, starting with carburated engines. But all the principles apply to fuel injected engines.