Gasoline engines are more efficient at torque peak, BUT wind resistance increases with the square of velocity. It takes about four times as much power to go 60 on level ground as it does 30, not just twice as much. The slowest speed at which the transmission stays in OD gear with the torque converter locked is probably going to yield the best mileage. If the engine is laboring at too low an rpm for the load, the trans will first unlock the converter clutch before it actually downshifts a gear. Converter slippage alone is worth five percent or more.