Torque is king, a motor that makes 400 ft-lbs at 3000 rpm accelerates a car exactly the same as 400 ft-lbs at 6000 rpm even though the second is making twice the horsepower. So twice the horsepower but exactly the same force. Where the high HP is effective is if you make the torque early and hold it to those high rpms. But if you are just going to make 400 ft-lbs from say 2000-4000 or 4000-6000 they will perform the same even though one motor has much higher hp. So you are right you could put a ton of gear in the big hp motor and then drive around at 4000 rpm getting 3 mpg.