Hmm... I'm interested in hearing why. (not doubting you, just interested in more info)
The gear ratio is the final step in the power step process. The transmissions are the same, so it seems to me that it should be the "10 spd bike" idea- Larger ration means drives easier, but at the same speed the engine has to turn more than a smaller ratio. If the engine is turning more revolutions, then your going to use more fuel.
This would be a benefit to towing, as a higher ratio makes it "easier" to pull additional weight. However, this higher ratio seems to me that it would also run the engine at slightly higher revs for the same speed, therefore using more fuel, and losing fuel efficiency.