How weight and rolling resistance drive fuel consumption
A car's fuel consumption is largely determined by the energy needed to overcome rolling resistance and air drag. Both increase with weight: heavier vehicles have higher rolling resistance as their tires deform more under load, and they require more power to accelerate to highway speed. The relationship is roughly linear: doubling a car's weight approximately doubles the energy needed per mile.
This is why larger vehicles burn noticeably more fuel. A full-size SUV weighing 5,000 pounds will consume roughly 50-60% more fuel than a sedan weighing 3,000 pounds, all else equal. The difference is measurable at the pump: a sedan might achieve 30 mpg while the SUV achieves 20 mpg, a gap that compounds over the car's life into thousands of dollars in extra fuel costs.
The mass penalty compounds with payload and size
The weight penalty extends beyond the vehicle itself. A roof rack, cargo carrier, or extra passengers increase the total mass and thus the fuel burned. This is why towing is fuel-intensive: a truck burning 15 mpg without a trailer might achieve 10 mpg while pulling a loaded trailer, the extra mass and aerodynamic drag combining to cut efficiency by one-third.
Modern vehicle design faces the challenge that safety regulations and consumer preferences push toward larger, heavier vehicles (SUVs and trucks now dominate sales), while efficiency regulations push toward weight reduction. Manufacturers respond by using lighter materials, smaller engines with turbocharging, and electrification to offset the rising mass of modern powertrains and safety systems. Without these countermeasures, the average vehicle fleet would be substantially less efficient than current reality despite the shift toward heavier vehicle types.