Find the torque curve graph for your engine. Now find a hill with a constant grade and try several gears at he torque peak rpm. One of those gears will give you the best mpg near the torque peak.
The torque peak represents the rpm where your engine delivers the most useable power. Depending on the compromises made by the manufacturer, your fuel/air mix at this rpm is close to ideal for power.
But we sre not seeking to optimize horsepower in hypermiling. We want efficiency. Maximum efficiency is obtained by leaning the mixture found at the peak of the torque curve down to the point where you flirt with predetonation on gas engines.
Most gas engines are designed to run slightly rich in normal use. This keeps them from being damaged from too hot a burn temperature or too extreme downforce generated too close to TDC - or both, which is really the case. This overly-lean mix results in melting of the piston dome.
Knock sensors, exhaust gas temperature gauges and realtime exhaust gas monitors are useful to people playing on the edge of this envelope. People messing around with turbochargers and superchargers tend to discover these limits quicker.
Ceramic piston head coatings are a way we can push these limits because they can withstand higher temperatures. Federal-Mogul makes these for various engines.
Ceramic coatings are mostly used for racing. It would be interesting to see ultra-lean mpg applications using them. (I think some Japanese engines have already incorporated ceramic coatings - does anyone know?)
For more info, see
Inside Secrets of Racing Coatings by Swain Tech for Automotive Coatings including engine piston coatings, ceramic coating services engine, automotive ceramic coating, ceramic header coating, dry film lubricant, race engine coatings, exhaust header co