Fuel Economy Tip – Fill Your Tank All The Way

Today’s tip is in regards to several emails I have received about filling up your gas tank only quarter or half way.

Fill up your gas tank all the way each time you go to the gas station.

The logic behind filling up your tank only a quarter, half or three quarters of the way isn’t faulty – by putting less gas in your car you’re reducing the amount of weight your vehicle is carrying around, thus increasing gas mileage.

While the logic isn’t faulty, the practice of not filling up your gas tank all the way more than likely is.

By not filling up your gas tank all the way, naturally you will have to take more trips to the pump, needlessly wasting gas.

Additionally, every time you remove your gas cap to add fuel to the gas tank, vaporized gas escapes from your tank. As I stated in a previous Fuel Economy Tip, over 147 million gallons of vaporized gas escape from gas tanks each year, so the less often you remove your gas cap, the better.

So, I would say that at the very least, the amount of money you save by driving around less weight is negated by the money you’ll lose by making extra trips to the pump and vaporized gas you’ll lose each time you remove the gas cap.


  1. “naturally you will have to take more trips to the pump, needlessly wasting gas.”

    Not “naturally”, plan your trips (A tip of yours I believe). The station I use is one a corner I pass everyday, pull in and fill up added distance 50 ft maybe. No reason you can’t do the same.

    “As I stated in a previous Fuel Economy Tip, over 147 million gallons of vaporized gas escape from gas tanks each year, so the less often you remove your gas cap, the better.”

    How many cars are there on the road? With over 200 million vehicles in the United States (assuming your stat of 147 million is for the US only, is it?) That’s less than a gallon per car per year, .73 gallons/car/year.

    That’s about .015 of a single gallon per fill up assuming weekly fill ups. That isn’t a ratio like 100lbs = 1% improved efficiency.

    SO let’s see:

    Fill to half tank: You go to fill up, you lose .015 gallons. Average gas weight of quarter a 10 gallon tank (40 lbs.) You lose a single .015 gallon by evaporation but gain a TOTAL of .03% for weight savings. NET of .015 gallons.

    Fill to full tank: You go to fill up, you lose .015 gallons. Average gas weight of ½ a 10 gallon tank (80 lbs.) You lose a single .015 gallon by evaporation but gain a TOTAL of .02% for weight savings. Net of .005 gallons.

    Bigger tanks only add to the savings.

    The above is WORSE case assuming as you have that that: ALL the evaporated gas is lost at time of fill up, and not lost over time due to lose gaskets (Untrue or you wouldn’t have used it as a previous tip). Nor does it consider the fact filling your tank up more means MORE evaporated gas/air into the outside air as the new liquid gas displaces it in the tank.

    You really need to do the math/research on these things.

    • houston says:

      Personally, I capture as much of the escaping vaporized gas into a plastic grocery bag and then force it back into the tank after I am through filling up. But don’t worry, I hold it in my other hand while I have a smoke while my car is filling up…I wouldn’t want to do anything dangerous.

  2. I completely agree with the person above and from my own experience I do save money only filling up half way.
    When I have a full tank the first half of gas will drop in a couple of days, the second half however can last me a week. I don’t have to go out of my way looking for stations to fill up, wasting mileage as I fill up right next to where I work. I would much rather making the savings stated in the comment before me than the ones you claim work.

    • houston says:

      Have you checked the actual amount of fuel or are you going by your gas gauge? Rarely is your gas gauge accurate enough for you to know exactly when you are at half a tank.

      • Doesnt matter what the gauge says, do the math

        1. – Fill up the first time – note the litres.
        2. – Drive till empty – note final mileage.
        3. – Fill to tank exactly 50% of first fill up – again note litres
        4. – Drive til empty – note final mileage

        Now compare which tanks got you the best mileage by volume.
        Try same experiment with diverent grades of gas.
        Conditions have to be the same while doing this so make sure your driving habits are consistent, ie city vs hwy driving, also oil and water topped up and tires evenly inflated.

        Half tanks win out in the end but the savings arent significant.

  3. is it true that vapourisation is more in a half tank than in a full tank

  4. houston says:

    I think you are missing the point when it comes to not filling up your tank. Unless your time is worthless, there is no savings to doing anything less than filling your tank, even if your regular station is on one of your normal driving paths. Heck, the extra time usually isn’t even worth it when it comes to driving across town to get a slightly better gas price. My mom drives to Costco to save $0.10 a gallon…but has to drive 5 miles to do it. I won’t even bother with the math of how saving approx $1.60 per fill-up easily offsets the extra $0.50 she pays for gas to get there. Even if she were to be there for other shopping, filling her tank when it is less than half empty is just a blatant waste of time. Spending an extra 5 minutes at the station for each fill-up because you are filling twice or even three times (with partial tanks) to go the same number of miles as you could on a full tank driven to empty is not my idea of a good way to spend my time. Sure, 5 minutes isn’t much…but add them all together and you are getting into some real time. If that is coming out of your TV watching, fine, but if it is coming away from your family, your friends, your volunteering, your work, etc…than it could be costly.

    That all being said, in earthquake country it is crazy to run your tank anywhere past half empty. When the big hits, a half-tank of gas might last as long as the gas shortages. With the economic turmoil around the world I would probably suggest that nobody runs past half a tank or else keep good gas storage on hand in your garage. (I have been having a hard time fighting my bad habit of running down to my gas light before filling up.)

    Does your “over 147 million gallons of vaporized gas” represent vapor gallons or unvaporized gallons? 147 million gallons of gas is a HUGE number…but the equivalent number of the gallons that once vaporized would represent 147 million gallons is much smaller. When vaporized, one gallon of gasoline takes up around 154 gallons of space. That would mean that if your 147,000,000 number is vapor it represents about 1 million gallons of liquid gas…which is not nearly as sensational of a number, clearly.

  5. Filling your car half full is beneficial on highway road trips. If you are like me and the wife and kids have to stop every 200 miles to use the bathroom it would be beneficial to use the bottom half of the tank, especially if you drive a vehicle with a large tank.

    I do not think it is beneficial to drive back and forth to work on a half tank. Added stops, starting the vehicle, opening the gas cap, not to mention added wear and tear on your vehicle.

    Save money by taking advantage of credit cards that offer $0. xx of per gallon and pay it before it collects interest. Also some supper markets offer $. 00 for using there store. Yesterday I had $0.70 off a gallon from King Soopers . Filled up the SUV and truck. Got 39 gallons for a savings of $27.30

  6. “is it true that vapourisation is more in a half tank than in a full tank”

    theoretically yes this is correct. there is a higher pressure in a full tank of gas versus a half tank. (technically the higher pressure forces more fuel vapor to condensate/more gas to remain in liquid state)

    however if penty’s calculations are correct then the amount of fuel vapor lost per visit is pretty much negligible therefore the difference in volume of vapor in full tank vs. half tank doesn’t really matter

  7. I love how financially comfortable people quibble over tiny amounts of money saved per month–and then congratulate themselves on how well the manage their money ( usually accompanies by comments about how if only so and so could learn to economize in such a way they could pull themselves out of poverty.) Ha! In order to “economize” like this you have to HAVE money in the first ace. Poor people don’t have the luxury of saving money in the long term because they are constrained by the demands of the present (ie, you only have as many choices as dollars in your pocket). If you REALLY needed the savings, this whole debate wouldn’t matter because you would be forced to fill up with whatever money was available whenever the need for gas trumped the other 1000 needs you had ( food, rent, etc)

    • All this talk about evaporation and no one has mentioned that all gas tanks have breather so the fuel is going to evaporate without the gas tank lid removed

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