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Increase Gas Mileage with Gas Tank Full or Half Empty?

July 24th, 2008 · No Comments · Budgeting

by Staff

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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