Five properties of physics that affect your gas mileage

Five properties of physics that affect your gas mileage
Credit: John Moreno/Argonne National Laboratory

Physics is inescapable. It's everywhere, making your Frisbees fly, your toilets flush and your pasta water boil at a lower temperature at altitude. We've harnessed these forces, along with chemistry and engineering, to build a marvelous contraption called a car—but many of the same properties that allow you to fly along the freeway also affect how much gas mileage you get out of your car. We talked to Argonne transportation engineer Steve Ciatti to explore some of the forces at work in your engine when it's on the road.

1) Vapor pressure

In summer, gasoline companies produce a blend of gas with lower , which basically means it is less likely to evaporate. Liquids evaporate more quickly when it's hot, so in order to prevent the gasoline vapor from contributing to summer smog and ozone pollution, the U.S. Environmental Protection Agency orders companies to change the formula.

The reformulated gas is cleaner and gets slightly better mileage for your car. Why? Gas is made up of a mix of molecules—all in the same family, but some short and some long. You get energy by breaking the molecules apart. Short ones, like butane, have less energy, and they cost less (so it makes sense that a company would want to add more of them). The part the EPA cares about is that short molecules also evaporate more easily, contributing to pollution. So in summer, the EPA restricts how many short-chain molecules can be in the blend, and your mileage increases because there's more energy in the gasoline overall. Unfortunately, it also makes the gas slightly more expensive.

2) Friction

Scientists at Argonne's sister national lab, Oak Ridge, tested cars' fuel economy at speeds over 50 miles per hour. For each extra 10 mph over, you lose a little over 12 percent of your miles per gallon. That increases as you go faster. Going from 70 to 80 mph costs you 15 percent, not 12.

Depending on the make of your car, it could be more or less. Some cars dropped as much as 25 percent.

"If you're driving at a steady velocity, all the power you're using is going into overcoming friction," Ciatti explained. "That equation increases by a power of three as you increase speed. So keeping the car going at 80 mph is using eight times the power you'd be using at 40 mph."

The faster you go, the more gas you'll need to move the car over the same distance.

3) Drag coefficient

The drag coefficient of your car is basically measuring how easily air goes around it. "You want as little frontal surface area as possible. If the car is a box, that's bad," Ciatti said. (You can see the coefficients of various shapes in the sidebar).

You can demonstrate this effect yourself, Ciatti said, when you hold your arm out the window while you're driving on the highway. If you lay your hand flat, parallel to the ground, the force isn't too bad. But if you hold your palm out, facing front, exposing more surface area to the direction of travel, the force is much stronger. That's the difference drag coefficient makes.

Automakers today pay close attention to , designing cars so that air slips easily around them. Choosing a car with an aerodynamic front—compared with a boxier make—will mean its gas mileage tends to be better.

4) Momentum

Weighing from 1,800 pounds (Smart Cars) up to 5,000 pounds and more (SUVs), cars have a huge amount of mass, which you can use for good or ill. It takes a lot of power to get an object of that mass moving, but once it does, you can use the momentum to coast—especially during city driving, with frequent stops and starts.

"One of the worst things you as a driver can do for your mileage is jam on the gas as soon as the light turns green," Ciatti said. "The harder you accelerate, the more power you need, and that all goes to waste as soon as you hit the next ." A savvier driver eases off the gas and relies on momentum to carry the car forward, especially if there's a red light coming up a block ahead.

"Dampening those jackrabbit starts will significantly improve your fuel efficiency," he said.

5) Rolling resistance

Remember when we said that if you're driving at a steady speed, most of the energy you're using is going to counteract friction? The tires are where that happens, and how much power it takes is dictated by a property called rolling resistance. Essentially, the softer the tire is, the more effort it will take to push it across a surface.

Why don't our cars have solid, hard tires, if that would be better for gas mileage? Harder tires are more efficient, but they provide less braking force, especially in rain and snow (because there's less friction) and they don't absorb shock as well. That's why your back might be sore after a ride in a performance car, Ciatti said; the tires on sports cars are built harder so that they can turn more crisply, but without that cushion, the ride quality is rougher. As a result, personal vehicles are engineered to balance all these factors—safety, efficiency, ride quality, durability—for everyday use.

You have some control over rolling resistance by inflating or deflating your tires. The manufacturer's recommendation takes performance, efficiency and ride handling into account; keeping your tires within that range will increase your gas mileage, Ciatti said.

Bonus! Air temperature

By far the biggest difference that air temperature makes is whether it makes you turn on the air conditioning, Ciatti said.

"The A/C is a power guzzler," Ciatti said. "Evaporating and condensing, which is what's going on in air conditioning, is horrifically power intensive." Yes, it's worse than opening the windows. Because the windows are on the sides of the car, they don't change the shape of the front of the car, which is what makes by far the largest difference in wind resistance. But you do drain engine power by running the A/C.

"Sometimes I'll turn the A/C off for a minute if I know I'll need to make a sudden acceleration, like a left-hand turn at the end of a light, because you can absolutely feel the difference in the amount of power you can get from the engine," Ciatti said.

Why is cooling a car so much more -intensive than heating it? Engines make heat as a byproduct anyway, so the heat is "free"—just blow air past the engine coolant.

Where the rubber hits the road

The term physics properties makes it sound inevitable, but "driver behavior is a huge, huge factor in how good your is," Ciatti said. "Jackrabbit starts, driving at extremely high speeds on the highway—those are the best ways to burn a lot of gas."

That enormous variation due to driving styles is why, when Argonne engineers and researchers test vehicles at the laboratory, they use a computer to drive the . They set the vehicle up on a dynamometer, which is essentially a treadmill for cars, and run it while they measure everything from emissions to battery life in hybrids. "This lets us control all the variables we possibly can," Ciatti said, "and driver behavior is a big one."


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Jul 04, 2015
"One of the worst things you as a driver can do for your mileage is jam on the gas as soon as the light turns green," Ciatti said. "The harder you accelerate, the more power you need, and that all goes to waste as soon as you hit the next red light."

Ah, yet another person that is a physics illiterate. E=1/2 M V^2!!!! The rate of acceleration has nothing to do with the amount of energy needed to attain a certain speed.

Jul 04, 2015
"4) Momentum

Weighing from 1,800 pounds (Smart Cars) up to 5,000 pounds and more (SUVs), cars have a huge amount of mass, which you can use for good or ill. It takes a lot of power to get an object of that mass moving, but once it does, you can use the momentum to coast—especially during city driving, with frequent stops and starts.

Yet another illustration of a poor understanding of Physics. Momentum is your enemy. The lighter the car the better it does in stop and go driving since less energy get dissipated by the bakes.

Jul 05, 2015

Ah, yet another person that is a physics illiterate. E=1/2 M V^2!!!! The rate of acceleration has nothing to do with the amount of energy needed to attain a certain speed.

The gas savings has nothing to do with accelerating as a source of producing kinetic energy. It has to do with the efficiency of the motor. When they specify the horse power of your car, that's the maximum horsepower. That's when you're reving somewhere around 5000 to 7000 RPM, something you would (or should) never do. Sure you're getting the most power from your car, but it's also the least efficient.

A fast acceleration forces your car to utilize the available horsepower, and especially in an automatic transmission, keeps it in a lower gear but revs the engine much higher. Sure you get far more acceleration, but if you accelerate slower, you never use the maximum horsepower and you never waste fuel.


Jul 06, 2015
A fast acceleration forces your car to utilize the available horsepower, and especially in an automatic transmission, keeps it in a lower gear but revs the engine much higher. Sure you get far more acceleration, but if you accelerate slower, you never use the maximum horsepower and you never waste fuel.


Actually, gasoline engines are more efficient at wide throttle because they experience the least pumping losses. A short brisk acceleration up to speed is more efficient than slowly creeping up to it.

But only if you already have some speed.

The power you add to the wheels is a product of angular velocity and torque, and while the car is at a standstill, torque is great but speed is zero, i.e. your efficiency is practically zero to start with and gradually climbs up as the wheel starts spinning.

The more power you try to use at the lower speeds, the less efficient you are at adding kinetic energy to the vehicle.

Jul 06, 2015
It's a bit difficult to keep track of where the power is actually going in the scenario, but the main point is that keeping high torque in a situation where the wheels are spinning slowly forces the engine and transmission to operate in an inefficient state.

The point is that a car engine is not like a wind-up toy. It doesn't have static torque stored up in a spring - it has dynamic torque, which means it needs to constantly use power to maintain torque. If that torque is not producing any power at the wheel, because the wheel isn't spinning yet, then the power is going out the tailpipe and the radiator, and grinding at your clutch / torque converter and/or other parts of the transmission.


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