What Increases Gas Mileage: Driving with Your Gas Tank Full or Half Empty?

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If you’ve been looking for ways to save money on gas, you may have heard that you get better mileage with a half-empty tank. Other people insist that you get better mileage when you fill up all the way.

So, which is it? Do you fill up all the way or leave your tank half-empty? Does it even matter? Let’s talk about the reasoning behind both methods, and why they might – or might not – work. We’ll also talk about some other ways you can improve your gas mileage.

Filling the Tank All the Way

Filling your tank all the way is more convenient than filling it halfway. But does it save you money? The reasoning makes sense at first glance.

As the tank drains, there’s space above the gas, and that space fills up with fumes. But gas vapor is explosive. Having a tank full of it poses a serious safety hazard. Automakers design their gas tanks with vents to prevent it from collecting.

The vents allow air to enter, which constantly flushes out vapor. Your car is safe, but some fuel is wasted. The idea is to keep the tank as full as possible to minimize loss.

So far, so good. But how much gas are you saving? To find out, I turned to a paper from the Environmental Protection Agency, detailing a study they did on evaporation rates in gas storage tanks.

I’m about to throw some numbers at you. Keep in mind that some of these are averages. Gas tanks come in different tanks and sizes, so your mileage may (literally) vary.

At 60 degrees Fahrenheit, a gallon of regular gas weighs approximately 6.15 pounds. If you flip through to page 87 of the paper, you’ll find the numbers for an 8,400-gallon storage tank. Over 12 months, this huge tank loses 48 pounds to evaporation, or a hair under 8 gallons.

Now, you might be wondering what an industrial storage tank has to do with your car’s gas tank. As it turns out, these tanks are also vented for safety. If we do the math, 8 gallons per year out of 8,400 is 0.0952% of the tank’s volume.

For complicated engineering reasons, the evaporation rate doesn’t depend on the surface area of the gas. In other words, the number depends only on the volume. So we can apply that same rate of 0.0952% to your gas tank.

The average American uses 656 gallons of gas per year. So, let’s do the math. 0.0952% of 656 gallons is 0.625 gallons. That’s how much fuel you would save per year by topping your tank up at every opportunity.

Now, let’s say you live on the road and you drive three times as much as the average American. You’re still saving less than two gallons of fuel per year.

That’s an insignificant amount of savings. It’s certainly not worth obsessively refilling your tank all the time.

Filling the Tank Halfway

The argument for filling the tank halfway is also intuitive. The more weight you’re carrying, the worse your gas mileage. Fuel adds weight, so carrying less fuel is more efficient.

Again, there’s some truth to this. But how much fuel do you save? Let’s run the numbers for a typical 15-gallon tank. If you leave half the tank empty, you’re saving 7.5 gallons of gas. That’s 46.1 pounds of weight.

According to the EPA, the average car loses 1% of its fuel efficiency for every 100 pounds of cargo. Extrapolating that, you can estimate a gain of 0.46% by dropping 46 pounds of weight.

Now, let’s go back to the average American, burning their 656 gallons of gas. Multiply that by 0.46%, and you get a savings of 3.02 gallons per year.

How to Increase Your Gas Mileage

As you can see, the people who advocate filling your tank halfway are technically correct. But let’s be serious. Saving three gallons of gas per year isn’t going to transform your finances or save the planet.

Once again, you’re looking at a lot of wasted effort. If you only fill up halfway, you have to stop for gas twice as often. Instead, let’s talk about some more practical ways to improve your mileage.

Cut Down on the Cargo

If you’re like most people, there’s some gear in your trunk. Some of this stuff is essential, like a jack or a gym bag. But ask yourself how much unnecessary equipment you’re carrying around.

For example, a lot of people drive around with sports equipment that they only use once a week. Why carry the extra weight? Take it out, and load it up when you need it. The same goes for anything else you leave in the car as a matter of convenience. Remember, every 100 pounds saves 1% of fuel efficiency.

While you’re at it, think about anything you’ve got mounted on the outside of the car. Are you driving around with a bike rack or cargo box? These add weight, and some of them are quite heavy.

If you need a cargo rack or bike rack, spring for a rear-mounted option instead of a roof-mounted one. Anything on your roof adds wind resistance as well as weight. Even a little drag can be as bad as hundreds of pounds of weight.

Reduce Idling

If you own a newer car, it probably has a stop-start system that shuts off the engine when you come to a stop. When you release the brake, the engine restarts automatically. If you don’t have a newer car, you might want to do this manually.

The reason for this is twofold. First off, if you’re idling for more than a few seconds, you burn more gas than you would by restarting your engine. Second, your engine runs at poor efficiency when it’s idling. You give off more pollution at a stoplight than you do on the highway.

Keep Your Tires Inflated

When your tires are underinflated, your wheels require more energy to turn. This means you’re burning more gas. Not only that, but poorly-inflated tires wear down more quickly. To find out the correct pressure level for your car, check the inside of your driver’s side door jamb.

Drive at a Reasonable Speed

Most cars reach peak efficiency between 45 and 50 miles per hour. That’s a nice sweet spot between a 25 MPH residential street and a 70 MPH highway. The faster you drive, the worse your fuel efficiency gets. If you have a lead foot, use your cruise control to keep your car at a reasonable speed.

Aggressive acceleration and hard braking also impact your efficiency. Speed up and slow down smoothly, and you’ll get better mileage.

Maintain Your Vehicle

Dirty fuel injectors, clogged air filters, and faulty spark plugs can all impact your fuel efficiency. Take care of your car and perform all the recommended maintenance. Not only will you get better mileage, but your car will run more smoothly.

Conclusion

Filling your tank halfway or all the way doesn’t make much difference. You might as well put a little desk fan on the back of your car to give yourself a boost.

Instead, take more practical measures to improve your fuel economy. You’ll save money, and the planet will thank you.

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