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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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.
Thanks, Bill. Even more reason why this type of thing is urban myth.
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
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?
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
Best comment we’ve ever gotten. Thanks.
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.
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.
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.
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.
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.