An engine spinning at 3000 rpm at an a/r of 15:1 will burn more fuel than that same engine with the same a/r at 2500 cruising on a flat highway road. If you get the same or better mpg with a numerically higher gear, it's probably because you have too rich of an a/r in the transition or idle circuits. Torque curve and efficiency have little to do with it unless you're towing or driving in hilly condition. Why do you think today's cars have over drive transmissions? My late model van turns 2000 rpm on the freeway at 70 mph, no where near it peak torque.