[jcp123]

As a rule of thumb, lower rpm tends to be more efficient, but a rule of thumb is all I take that as. There are tons and tons of variables such as airflow through the engine, throttle needed to keep your speed, and many many more which make it vastly more complicated than any rule of thumb. Every engine will have its sweet spot(s) where it burns the least amount of fuel per hp produced (the guys with lots of university degrees hanging in their offices will call this Brake Specific Fuel Consumption). If low rpm were always the key, we'd have engines which were capable of merely idling along at the speed limit. Sorry I can't speak specifically to your car, but that's the general reason why you indeed might not always see top MPG when you're at your lowest rpm.