part 1. will you get the best fuel mileage (MPGS) at lower rpms, ignoring factors such as the speed of the car and aerodynamics. lets just say the car is kept in first gear. so will your engine get the most MPGs at lower rpms? does the gear your in effect MPGs? i know the higher the gear the faster the speed of the car so more drag and less MPGs but what if you were to put the car on a dyno and have the wheels moving but not the car, would 1000 RPMs in 1st give you the same fuel consumption as 1000 RPMs in 5th. also please note this is about Miles per Gallon, not fuel efficiency in terms of gasoline being converted into power. part 2. if an engine will get the most MPGs running at lower rpms what happens when your vehicle is put under a load and struggles at said rpms. for example assume your engine is most efficient at 1000 RPMS (lets just say this was found out when your vehicle was in neutral or drive wheels had no load on them) but at this RPMs in first gear your car struggles and sputters to keep this RPMS. even though the engine is at 1000 RPMS its struggling, will this return the same MPGs, again ignoring vehicle speed.