when driving any car from 3 cylinder lawnmower to v12, as soon as you put load on it, then it will cosume more fuel.

load is basically pushing the pedal down more then required.

the trick is to get the car up to the desired speed with a moderate amount of gas, once there keep the acceleration within the rev range (example 2500 to 3500) and you should be fine.

the more you put load on it at such a low rev or high gear, the more it will drink and this fuel will go straight through the exhaust buddy.