Here’s a question for you: can you improve your car’s gas mileage by only filling its tank half-way? Or should you fill your tank completely each time?
Since I’m a physicist, I might as well make some back-of-the-envelope calculations and use them as the basis for sweeping generalizations. If you’re smart you’ll check my work.
On the face of it, this is an easy question. Fuel adds weight, and the structure to hold that fuel adds even more weight. When you’re working against gravity by going up hills or trying to get a car moving from a stop, that weight works against you. You have to do more work and thus consume more energy. Rockets get around the problem by throwing away parts of their gas tank as they fly, like how the Saturn V rocket used on the Apollo missions had separate stages that fell off as their fuel was used up. It’s part of why single-stage-to-orbit (SSTO) rockets are hard to design.
You can’t lop off bits of cars like you can with rockets. Instead, to boost fuel economy you minimize gas tank size, which is why small cars like the Honda Fit and Toyota Yaris have around an 11 gallon tank. But once you’ve got your car, you’re stuck with your gas tank, so the only thing you can change is how much gas you put in it. You’re definitely better off with a half-tank for stop-and-go and for a lot of hilly driving. It’s possible that, for long drives at near-constant speed on a straight highway, greater mass will help you maintain your momentum against friction.
Really, though, the effect is small enough not to matter. Consider a Ford Taurus, which has a 20 gallon tank. Gasoline weighs around 6 lbs per gallon, so only filling it up half-way saves you 60 lbs. The car itself weighs around 3600 lbs. You’ve decreased its weight by just over a percent. If you assume that roughly translates into an equal increase in its MPG1 (which is around 22), then you’ve increased its MPG to 22.4. If you drive 12,000 miles a year, and gas costs $4 a gallon, you’ve saved yourself around $39. Meanwhile you’ve had to stop to fill up an extra 26 times, costing you a lot more time for that 40 bucks.
What about running your air conditioning versus having your windows down? That’ll most likely depend on your speed. The A/C decreases your gas mileage. At city speeds of 35 MPH or so, you’re probably better off with the windows down and the A/C off. As you speed up, though, the drag on your car due to the open windows increases, to the point that the MPG hit is worse than what you get from the A/C. What’s happening is that your car is aerodynamically designed to keep the air flowing uninterrupted over its surface, assuming you’re not driving an H2. When you open the window, a chunk of the air comes into your car. A lot of that air pushes on the back windshield, adding drag. It’s like when you stick your hand out the window of a car and turn it so your palm is facing forward. You’re also creating turbulent whorls of air around your windows. That’s bad, because when you go from a smooth airflow to a turbulent one, you have to work harder to push through it. The faster you’re going, the worse that effect is. So at low speeds, the MPG hit for open windows is likely lower than the MPG hit for A/C, but at higher speeds, it’s worse.
Honestly, though, that too won’t likely make much difference. The real MPG killers are how much you accelerate from a stop and how fast you drive. Making your car move in the first place takes a lot of energy, which is why stop-and-go driving in the city leads to lower gas mileage than highway driving2. And after a certain point, the faster you drive, the more your gas mileage drops, often by five or ten MPG.
So don’t worry too much about how much you fill your gas tank by, or whether you’re using A/C or rolled-down windows to keep you cool. If you’re worried about your gas mileage, avoid jackrabbit stops and drive more slowly.
1 This is definitely a zeroth-order approximation, but it gives me a decent ballpark estimate. It’s possible the actual effect is 2 or 3 times greater, but not an order of magnitude greater. (back)
2 Unless you’re driving one of those foofy Priuses that use regenerative brakes to recover some of that energy. (back)