As gruez says, a heavier car needs more fuel to accelerate and has more rolling resistance. It also needs more fuel to drive up a slope. Some of these can be re

As gruez says, a heavier car needs more fuel to accelerate and has more rolling resistance. It also needs more fuel to drive up a slope. Some of these can be regained in a car with regenerative braking, but even than only partially because there are always losses. But the difference is not nearly as large as it is for airplanes. Having your fuel tank only half full means more trips to the gas station, which might even negate all gains you try to make. It seems a good idea though not to drive around with things live heavy tools all the time when you don't need them.