A Brake Specific Fuel Consumption map would help answer the question. Good luck finding any, unfortunately. For the example above, the red area is the highest efficiency area. Efficiency goes down as you move into the darker colors. Say this car needed 30 hp to propel itself 60 mph. Go to 30 on the right hand axis and follow the blue curved line. You can see that something around 1700 rpm is the most efficient area for making 30 hp. As you can see this is very heavy throttle, as you'll be asking ~93 ft lbs at an engine speed where ~110 ftlbs is wide open. Fuel usage is about 0.411 lbs/hp-hr (or about 1.9 gallons per hour, or at 60 mph, about 31.6 mpg). You could also run at 5,000 rpm and make the 30 hp necessary with much less engine load, but efficiency then falls to the 0.657 lb/hp-hr range. That means about 3.03 gal/hr, or 19.8 mpg. Generally most engines follow this trend, that is, best efficiency at high load, low rpm. Now tuning and volumetric efficiency come into play as well, so it can't just be said "WOT at 500 rpm is best". Many harmonics go out the window and an engine might run with poor efficiency as rpm gets to low (valve overlap, reversion, lost intake charge, thermal efficiencies, and sooooo many more variables). Plus the way the engine is tuned as well. Lug into a detonation zone and you'll often find mapping that dumps in fuel and yanks out spark to keep the thing from knocking to death. So it's back to a sort of guessing game without a BSFC map, or just knowledge of the engine (first hand, or using something that has been around a while and has lots of users and information on it). Good luck.