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Increase Gas Mileage with Gas Tank Full or Half Empty?

July 24th, 2008 · 22 Comments · Budgeting

Kristy Welsh

by Kristy Welsh

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There has recently been a lot of talk on how you can get better gas mileage by driving when your tank is half full to empty or making sure you fill up when your tank is half full. Which is correct – or does either method hold any water?

The fill ‘er up when it’s half full foks proclaim that if you leave the gas tank empty, the gas will be evaporating in the empty space of the tank. Gas tanks are designed to force out all those gas fumes from the tank into the atmosphere and this gas is wasted. Yes, it’s true, but how much gas are you actually losing? To tackle this one, I woke up the engineering part of my brain (I used to be one) and looked at a paper published by the Environmental Protection Agency and their analysis of gasoline storage tanks and the evaporation rates.

At 60 degrees Fahrenheit, a gallon of vehicular gasoline weighs about 6.15 pounds. In the above paper on page 87 (if you want to chew through it), they go over what the gasoline vapor losses are for large storage tanks. For a 8400 gallon tank the total loss is 48 lbs (or about 8 gallons) a year.

Storage tanks are vented just as car tanks are. You could argue maybe more gas is vented out of your gas tank when you fill up constantly, but let’s say the evaporation loss factor for argument is 8 gallons per year/8400 gallon tank = .000952/year per tank. The equations for evaporation do not depend on surface area of the gas, meaning that the number is valid based only on the amount of the gas, not the size of the tank.

I read somewhere that the average American buys 1000 galllons of gas a year. Taking that number and our “evaporation loss factor”: if you fill use 1000 gallons of gas a year, you will lose 1000 x .000952 lbs or .9 lbs of gas PER YEAR. That’s about an 1/8th of a gallon of gas PER YEAR. OK, let’s say you think this number is too low by a factor of 10. Then you are losing 9.52 lbs of gas a year (or a little over a gallon) PER YEAR.

Therefore, in my opinion, the effects of how much gas vapor exists in your tank and is lost to the atmosphere when you fill up is insignificant.

The wait until the gas tank is almost empty before filling folks proclaim that you get better gas mileage due to less weight. Well, they’re right, but how much of an advantage? If you have a 15 gallon tank, that’s 92 lbs less of weight and your gas mileage will definitely be better. How much better? That is entire dependent on the car you are driving. If you’re driving a small car with a small engine, you will get much more advantage than someone driving a powerful car or truck with an 8 cylinder engine, the advantage will be much less.

I’m going to do some unscientific estimating based on my own driving experience. In my 2003 Honda Element, a car fully packed for vacation with 2 extra passengers (carrying I would say a total of 900 extra lbs) gets 3 miles less to the gallon. Normally, it gets about 25 with just me on the highway. My tank of gas takes 15 gallons, or 92 lbs.

Since the gas weight is about 10 times less than my overloaded passenger and luggage weight, I’m just going to divide by 10 and say my car gets .3 miles per gallon less when I am driving with a full tank over an empty tank. But how long can you drive with an empty tank? Let’s assume I could keep my tank half-full forever. Then I would get about .15 miles to the gallon better mileage (half of my reduced mileage when the tank is full), but I would have to stop constantly for gas, which is a waste of my time.

In any case, the savings due to better gas mileage is due to less gas weight is negligible, and the numbers I ran are pretty crude. Don’t take them as gospel.

Conclusion: A much better way to increase your gas mileage and one which will have immediate effect: keep the air pressure in tires at the recommended levels, keeps your car speed moderates and keep excessive braking to a minimum.

Have any of you seen improvement keeping your gas tank half full or half empty? Leave us a comment!

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22 Comments so far ↓

  • Bill

    Every car built for sale in the U.S. since about 1966 has a fuel tank evaporation emission control system. It is designed to siphon off fuel tank fumes, filter them through an activated charcoal canister and vent them to the air filter or the intake manifold. From there the fumes are sucked into the engine and burned.

    The only place where fumes can reach the outside air is when fueling up at the gas station. Many jurisdictions require gas station pump nozzles fit tightly over the auto’s gas tank inlet so that most of the fumes are recirculated back to the station’s tanks.

  • Kristy

    Thanks, Bill. Even more reason why this type of thing is urban myth.

  • adam

    personally i think the best way to decide this is a little “myth busting” of your own. i was getting around 21 miles/gallon with my ’04 subaru WRX. as of late, i tried filling up when i hit a half tank. for 2 fill ups i calculated over 24 miles/gallon. that’s good enough for me. so, i’ll take the results over hypothetical situations.

    granted, i was driving in different situations with my trials. i suppose if you want to remove some variables, next time you go on a long road trip let the gas tank go to empty on the way there. also, try and fill up using the same gas. ie Mobil, Exxon, or whatever. on the way back fill up at half a tank. i’m nearly certain you will see a significant increase when you fill up at the half tank threshold.

    also, in addition to making sure your tire pressure is correct, check your air filter every few thousand miles. have them check it when you get an oil change. many stations don’t check it unless you tell them to

  • shari

    :roll: It really doesn’t matter where you buy gas. Companies sell gas to each other all of the time. I used to work in product movement for a few oil companies so I know that just because you buy gas at Mobil it doesn’t mean they refined it. It could have been refined by Exxon or Chevron or an independent.

  • Wizard

    The ‘better mileage top half of tank’ is due mostly, in part, to a non-linear sensor in the gas tank. To make a sensor that would accurately represent real fuel levels, would be excessively expensive. You wouldn’t expect Detroit to do that would you?

    Gas Tank vs. Fuel Gauge
    The float’s range of travel is divided into 4 equal parts, with each representing the quarters, as displayed on the fuel gauge. As shown in red, the actual levels in the tank to not reflect accurately to what is displayed on the gauge. When the gauge reads, 1/2 tank, there’s actually around 1/3 tank.

    So the answer is NO! You don’t get better mileage from the first half of hte tank. You just burn through thwo thirds of a tank before your gauge reads one half.

    Source: http://www.wizardinthebasement.com

  • Kristy

    Best comment we’ve ever gotten. Thanks.

  • Wizard

    Why, thank you. A few years ago, I put 10 gallons in an empty 20 gallon tank (my truck has 2 tanks) and the gauge read 3/4. Not being one to let an unanswered question drive me crazy, I pulled the sensor out of the tank and did some tests on it. The graph is an illustration of what I found to be the case in my Ford F150. Since so many people ‘believe’ that they get better mileage in the top half, I assume this is the case in many other vehicles as well.

  • andy

    Kristy,

    Your evaporation calculation has a math error in it…off by a factor of 10. You have a conversion factor of 8 gallons lost per 8400 gallons in a tank per year, which is 0.000952 GALLONS lost per GALLONS in a tank. Therefore, multiplying 1000 GALLONS by your “evaporation loss factor” equals 0.952 GALLONS lost per year, not POUNDS lost. So, multiplying by a factor of ten yields about 10 gallons lost per year, which was about $50 in gas not too long ago.

    And storage tanks do not intentionally vent to the atmosphere. Why would they? They are all equiped with pressure relief valves or pressure discs in case of over-pressurization. But they are not open to the atmosphere. Why would you leave your fuel tank open to the water present in ambient air? I think the evaporation loss they are referring to is through leakage, not intentional venting.

    If you left a strorage tank of fuel open to the atmosphere, then you would evetually lose all your fuel to evaporation. Well, not all of it, but the liquid phase would reduce to an infinite smallness over a long period of time. There is always a vapor pressure present in a closed environment that is dependant on temperature. On hot days, the vapor present above the liquid would be more. Anyway, opening up the tank to the atmosphere would cause this vapor phase to be costantly vented. Eventually, you will lose all the gas in the tank.

    And we certainly wouldn’t leave our auto gas tanks open to the atmosphere. Getting water in our fuel lines is bad news in the winter time.

  • Chris

    The evaporation of fuel in the tank is amplified by the temperature, more so than that of a storage tank. First, fuel tanks are not double walled like many storage tanks (for spill containment) which adds some insulation, secondly many cars have a fuel return line which returns excess fuel to the tank. Usually by pressure regulator. So fuel going through a pump will heat up, also the fuel lines near a hot engine will absorb some heat and return it to the tank. Ever taken off the fuel cap and heard a hiss? This is excess pressure due to heat. A guess would be the temperature of a fuel tank could maintain 100 degree temps or more accelerating evaporation. The evap control system mentioned earlier pulls out those fumes for combustion.

  • Soumya

    To Wizard’s analysis,

    I guess most people measure the mpg by dividing the miles travelled after tank fill by the gallons filled in (from the receipt). The fuel gauge is a rough measurement of current status. The investigation is really good but seems not relevant to the current topic. Correct me if I’m wrong.

  • Drfitt

    I had just observed from my car computer that the MPG seemed higher at the beginning of a frequent 300 mile drive. I looked here to confirm my suspicion. The calcultions are interesting but real science has be off the paper. Driving here,Dallas, I averaged 33.4 MPG after filling up in Amarillo. Air psi was checked prior to departure. Air temp and weight will be similar on return. I’ll start out with about 1/4 tank on return. I’ll report back.

  • Jonny Switchblade

    In some cars the float is a completely linear system as I’ve seen first hand having worked on many cars with my father. With the evaporation retrieval, exhaust recirculation and other fuel salvage systems, our cars all produce different results under different conditions but in my opinion it’s pretty consistent that cars do better with a full tank and here’s why.

    Bottom line is, you fill the tank and check your receipt like I do and in comparison to your fuel gauge and tank size, you determine how good your mileage is.
    My car without question, gets much better fuel economy when the tank is full. If I drive like an old ninny, I can get 150 miles out of 1/4 of a tank that holds 16 gallons. When my tank hits half full, it’s generally right around 8 gallons to completely fill it back up.

    It’s also truly a myth that removing 100 lbs of weight is going to produce an improvement in fuel economy that isn’t negligible. You’re dealing with ROLLING weight. The extra fuel loss is only due to accelerating the car from a dead stop so if you fill a car with 4 people instead of 2 and drive 350 miles non stop, you’re not going to see a noticeable difference in fuel economy regardless of the difference in passenger load even if you do make a couple short term stops on the way.

    Make that reference where you’re driving stop and go for 2 hours and your car also isn’t benefiting from the air keeping it cool, your fuel economy is going in the toilet whether there is 1 of you or 4 of you. Then aspects such as aerodynamics, cruise control A.I. quality and weather conditions.

    Another bottom line is, gas DOES escape through evaporation, particularly after a lot of heavy driving then bringing your car to rest with half a tank or less. The expansion due to the heat allows for the gas to escape regardless of how well you believe your cap is sealing the tank. It’s slow and inaudible but it’s also steady and consistent. A hot car takes 4-5 hours to cool down with half a tank or less is going to be missing a gallon of gas, sometimes more the following day.
    I’ve been experiencing this one for decades in numerous cars I’ve owned. Come home, car is hot as hell, tank is about 1/4 or 1/3 of the way full and when I check the gauge the following day, sometimes a COUPLE gallons have vanished.

    So it’s certainly a debatable issue depending on your car but in most cases, you’re going to get the best mileage when your tank is closer to being full than it is to empty.

  • Rafael P. Santana

    For Johny Switchblade you may be right but I will not argue with your theory. Today’s car’s are extremely accurate and fuel can escape via evaporation even with a small consistent leak. However, today’s cars can detect an extremely small leak over a period of time. The computer onboard yor vehicle is constantly monitoring fuel pressre and adjusting and testing it as required, if it notices something out of the ordinary, ON comes your check engine light telling you about an evaporative emissions system failure sometimes even stating that it is a very small leak, I’ve seen this many times. It doesn’t matter how full or how empty your tank is, it WILL NOT AFFECT YOUR FUEL ECONOMY, fuel economy has everything to do with keeping your air/fuel mixture at 14.7:1 its lean if its got too much air and little fuel, its rich if its got too much fuel and little air, your fuel pump will always deliver the same amount of fuel under pressure to the fuel injectors, any fuel that evaporated in the tank is sucked out of the charcoal canister and burned in the combustion chamber, the computer even accounts for this by slightly leaning the mixture. The level of fuel in your tank does not affect fuel economy…it all has to do with what is going on inside the combustion chamber, how you drive your car, etc. Did you know that manufacturers of vehicles make up for brake drag in fuel economy? your pads slightly touch the brake rotors and this very faint drag is accounted for in fuel economy, that is why dragging brakes, an over-load car, even a small amount of weight affects how a vehicle performs, if its s small change you wont see the difference but if its a big change, you’ll see the difference…example: towing a trailer during a trip, you’ll get less fuel mileage because your engine is working harder pulling the load. Fuel economy has nothing do with how much fuel you have in your tank and if fuel was slowly evaporating away over a long period of time, the computer would catch the leak and flick on the light everyone hates “Check Engine”.

  • Pedro Farman ; Estados Unidos, Florida

    Mecanico de Ferrari de Miami Florida aqui, 43 años de experiencia en la profession. Siento mucho decir que la cantidad de gasolina en su tanque no affecta la economia del auto. Es cierto que la gasolina se esta evaporando, pero toda gasolina evaporado es guardada en el systema evaporativo del carro y luego es aplicada cuando el motor esta prendido. la bomba de gasolina siempre mantiene la misma cantidad de gasolina en pression a los injectores del motor…si la mescla continue mucho aire y poca gasolina, la mescla esta muy pobre, si la mescla contiene poco aire y mucha gasolina, decimos que esta enriquezida. El nivel de gasolina en su tanque no afecta la potencia del motor, el carro consumira menos porque menos peso requiere menos energia…cuando tengan esta discussion con una persona que reclaman que un tanque lleno resulta en mejor economia de gasolina, digale que esta incorrect y que vuelva a la escuela. conduscan con cuidado, desde florida, pedro a la orden!

  • Joe E

    I dont think it is about 1 person saving a million dollars at fill-up. It is a collective savings nation wide. The consumption fuel and giving the gas companies extra earnings. But as most I will drive an extra mile to save 2 pennies a gallon not to reward the higher priced pumps and know a penny here and a penny there will give me a few more pennies on a rainy day.

  • gd89

    Johnny Switchblade is right that the weight of gas in your tank is insignificant in regards to fuel efficiency, but don’t forget about friction. If the weight of the car were to double, the force required to maintain the car at speed (disregarding drag) would be doubled as well, and you would see a decrease in gas mileage. (The force of friction is equal to its coefficient, about 1.7 for tires on concrete, multiplied by the total weight). A significantly heavier vehicle will also have to deal with a stronger drag force. It’s just that the gasoline in your tank weighs so little in comparison to the actual vehicle, that it doesn’t make much of a difference. Having four heavy people in your car, however, is enough weight that you might actually see a small decrease in fuel efficiency, all other factors constant.

    All in all, don’t concern yourself too much with gas tank levels if you’re trying to save money– you’ll look pretty stupid if you run out of gas on the highway trying to bump your mileage up from 21.0 to 21.1

  • Rey@r-a-ones

    Johnny Switchblades’ explanation aggrees with my experience. I own a 2010 Didge GRVan3.3L Ist top quarter I get 200 to 225 Km. the next Quarter to Half Full…less KM…125 to 150 Km VS. 200 to 225km in the first Top Quarter.

    Any more similar experiences?

    Therefore, for me, I would refill again when the Gauge hits 3/4Full.

  • Rick Honeberger

    I drove 81 miles and used a half a tank of gas. I was pulling an empty 6×10 trailer. On the way back , ( same exact route) I used a quarter of a tank. First half tank was bought at a different station than the second half tank. WHY?

  • KP

    Rick,

    Could you have gone more uphill on the way there vs downhill on the way back (maybe unnoticed?)?

  • Quadbravo

    Seems counter intuitive that you’d get better gas mileage carrying more weight. The suggestion that gas evaporates faster in a tank only half full seems also pretty questionable. The surface area won’t really change until you’re near empty. I think most of this came from experiences on older cars. Pre 1973. Santana is right. Your gas is on lockdown today and any experience you have could well be due simply to weather, road conditions; heck, even different tires perform differently in different weather. If your car is less than 10 years old you’re chasing fly poop on this. If I stopped to fill every time I was at a half tank I would have made 200 more stops in any given year. Foregetting the lost time, the reacceleration cost alone would have offset any perceived advantage.

  • L.Razvan

    I have a opel vectra C and when i go full tank he lose gas … is this somthing normal or i need to replace my tank ?

  • KEN WU

    I have had 2 cars in my life and one thing I always do is keeping a log everytime I fill up. I do this because my grandfather started this habit of monitoring gas efficiency, he is a firm believer that gas efficiency can tell you how is your car doing. I have been doing this for over 12 years now. I can be very confident base on my finding that it is almost always true that you get much better gas efficiency if you wait until it is almost empty, my calculation gives me a difference of about 3 mpg on a Honda Accord V6. May not seem much, but for a 17 gallon tank(I fill it up when I used up about 16), it does make a difference. Also, if you refill weekly like I do, the saving does add up. for me, it would be about 2496 miles of savings a year or 124 gallons.

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