The information is found in a chart called a Power Load Chart or sometimes called the "Sea Level Performance Chart". The chart contains several curves, you are interested in the curve labeled "Propeller Load SFC" (Specific Fuel Consumption).
All type certificated Lycoming and Continental engines (and most likely all type certificated engines) publish this chart. Many modified and otherwise tampered-with engines do not; and this is a crime. The reason is, without a Power Load Chart one does not know the fuel consumption rate of the engine. Without this information, flight planning, range, fuel burn, endurance, cannot be computed. Sloppy data is the outwards sign of sloppy workmanship, but I digress.
This chart is in the "Engine Operators Manual", a small booklet published by Continental and Lycoming that contains a wealth of information. An example chart is contained below. This chart is also in the Mechanic's Toolbox, Horsepower Tool. Select "Power Charts" from the top menu bar.
The lower-most curve on the right is the one we're interested in today. Notice that as engine RPM increases, specific fuel consumption decreases. This is shown by the downward slope of the curve. At 2,400 rpm the curve starts to rapidly rise. For this engine, 2,400 rpm is the most fuel efficient rpm at .50 pounds per horsepower per hour. The other curves on the chart can be used to compute horsepower and with this information, total fuel burn per hour.
