Seems like a dumb/basic question; but, I want to totally flush out the answer.....as there's more to it than most people think. So, when I drive my SUV 250 miles to Las Vegas...I get 20 mpg. Many cars will get 30-35. What extra work is being done, and/or what inefficiencies are being experienced in my SUV versus the passenger car (and I'm focusing on the highway part...not the start/stop parts)? Here's what I've boiled it down to: * Acceleration- even if we ignore the energy required to accelerate from a stop, and only look at "highway" mpg.....a vehicle is still always accelerating between different speeds, even when you think you are "cruising" at a constant speed. The more you Δ speeds in a heavier vehicle, the more you're exposing the advantage of the lighter vehicle. * Climbing (accelerating against gravity)- the larger heavier vehicle must do more work when climbing a hill, assuming the same speed as the lighter/smaller vehicle. The more hills involved, the more the larger/heavier vehicle is being shown to be at a disadvantage. This is just another form of accelerating. * Aerodynamics- The larger vehicle (usually) must displace more atmosphere per hour (assuming same speed as the smaller vehicle). IF the coefficient of drag for the heavy SUV and the lighter passenger car are the same....then this will not matter....but that's a big "if". So, the weight of the vehicle itself doesn't automatically cause the problem (in terms of reduced highway MPG).....if you could maintain a constant speed and experience no hills, then you'd be more or less on par with a lighter vehicle. Right? In fact, two identical vehicles traveling at a perfectly constant speed (in theory), on perfectly flat ground.....but one with 1,000 Kg of bricks inside, while the other is empty, both should get roughly the same MPG over a 1000 mile trip. Yes?