The HP vs. Torque vs. Fuel Efficiency will always be confusing because of comments already made: HP is torque per unit time; HP=torque/sec with torque as ft-lb or N-m and HP as Ft-lb/sec. The metric equivalent to horsepower is watts as J/s or N-m/s. The two are related and only separated by time.
Torque is work or energy. Work per unit time (and energy per unit time) is Power. Power is in horsepower or watts.
Force applied for a distance is work; i.e. torque; i.e. energy: Torque = force * distance: ft-lbs (feet*lbs) and N-m (Joules)
What matters is the force needed over a distance, or energy, which affects gas mileage. Gas has energy and the conversion of this energy into motion takes force. It is this force that determines gas mileage fuel efficiency.
As an extreme example: a bumble bee and a Mack truck; which uses the least energy to start and stop? Since motion is related to acceleration, a, and force is mass * acceleration (i.e. F=ma) then the more massive object will always use more energy to start and stop. The bumble bee wins. Recognizing energy use determines miles/gallon or E=force*distance and thus for the energy in a gallon of gas, the distance = Energy/force. So, to increase the miles per gallon the force is reduced.
Reducing the force from acceleration, bearing drag, internal friction, aerodynamic drag, rolling resistance drag, etc. will increase MPG. Viewing MPG relative to torque and horsepower - which are both energy with and without a time component - will not lead to improving gas mileage. If the rider does not accelerate quickly and uses a fairing, the fuel efficiency will go up and a lighter motorcycle will get better fuel efficiency than a heavier one. Physics.