Can You Improve Gas Mileage By Filling Your Tank Half-Way?

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)

10 thoughts on “Can You Improve Gas Mileage By Filling Your Tank Half-Way?

  1. I’ve seen analyses that argue that removing, say, 100 lbs of junk from the car [:whistles innocently:] would get you 3-5%. Yeah, it’s not gonna get you very much.

    [You mentioned SSTO because you knew I was going to read this. I don’t know whether to be appreciative or to shake my fist in your general direction.]

  2. I’ve wondered about the whole filling the gas tank only halfway thing. It’s interesting to think about the numbers of it.

    And I think “foofy” is my new favorite adjective.

  3. A claim I saw in the movie theater last weekend: washing your car leads to a 4% mileage improvement. I can’t quite figure that one out. Is the dirt adding drag? Must I wax to reduce friction?

  4. My grandfather who is 86 this year suggested that if people really wanted to save gas and reduce waste then we’d lobby for a return to a much older speed limit standard.

    45mph max on the highways.
    35mph max on any non-highway road.

    We’ve gotten so wedded to the “fast and right away” I wonder if people can ever conceive of a less-so being acceptable.

  5. Paul, this is my skeptical face. 4%? Really? It’s the equivalent of removing several hundred pounds from your car? Washing and waxing your car will improve your aerodynamic profile, but not by that much.

    In doing a little digging, I see the EPA’s site on fuel economy estimates 2.6% of your car’s energy goes to overcoming aerodynamic drag. That number will fluctuate depending on how fast you’re driving, but I can’t imagine it going much above 4 or 5%, in which case they’re claiming that washing and waxing your car will completely remove the effects of drag. So, no, I’ll add this tip to the pile that includes “put magnets on your fuel line!” and “overinflate your tires!”

  6. I’ve decided that you must be a member of the Mythbusters staff. They did at least one episode on gas mileage and efficiency. They attempted to prove that driving with your windows up actually eliminates drag and saves gas. They too concluded that it was an inconsequential difference in the grand scheme of driving, therefore I enjoy my hot summer drives with the AC on. I will also add that I’m going to enjoy acting like a 2 year old by inserting FOOFY into every possible sentence for a while.

  7. I mostly agree with everything, especially the conclusions. In addition though to stopping for gas more often (and extra starts and miles to get the gas station) you also lose some probably trivial amount through evaporation and vapor. I don’t know if the recovery systems they added a few years ago actually recover it or just keep it out of the air.

    But add another easy thing to do to increase milage:

    Properly inflating your tires. And of course, biking or walking or carpooling.

    jv <– foofy prius driver.

  8. Perhaps a better timing strategy for buying gas is to wait till the price drops below some moving average then buying as much as the car will hold. This is what I try to do (on a gut estimate rather than a calculation of the moving average.) It works better with the freedom of timing afforded by low consumption.

    As for the optimal speed for AC, the rate of heat exchange through the radiator will also be improved at higher speeds. I just always roll the window down out of habit, except the window motor is now failing. (I just got the steering repaired, so the car trying to reestablish its equilibrium level of non-functionality.)

  9. I have a question, I have been aruguing with my cousin’s husband about gas mileage and how you should not put in 5.00-10.00 at a time to go here in there in a 1985 GMC Suburban., instead of putting in all of it at once to put more gas in like 30-40 dollars, which gives about 1/4 tank in my suburban. He says its less weight. But my truck weighs enough already and I believe that you need to put atleast 1/2 tank in it to get the best gas mileage. Can You Help Respond by email. Thank you


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