What affects fuel economy

Where the fuel goes

Only part of the energy in a tank of gasoline ends up turning the wheels. Much of it leaves the engine as heat through the exhaust and the radiator. What reaches the wheels is spent on four jobs: pushing through the air, overcoming the rolling resistance of the tires, lifting the car up hills and speeding it up. Braking throws away the energy you spent speeding up, which is why stop-and-go driving is so costly for conventional cars.

Speed and air drag

Air drag grows with the square of speed, and the power needed to beat it grows even faster. At city speeds drag is a small share of the load. On the highway it becomes the main one. US government guidance notes that mileage usually drops quickly above about 50 mph, though the exact peak varies by vehicle.

That makes highway speed one of the strongest levers you have. Dropping from 75 mph to 65 mph on a long trip usually costs only a few minutes and saves a noticeable amount of fuel. The example log on the home page shows the effect: the same car did far better at 65 mph than at 75 mph with a roof box.

Things that add drag

Anything that spoils the shape of the car adds drag, and the cost climbs with speed.

• Large roof cargo box at highway speedabout 6 to 17%A blunt box on the roof adds drag, and drag grows quickly with speed. A rear-mounted carrier costs far less.

0%Share of mileage lost, on a scale to 40%40%

• Roof boxes and racks. A rear hitch carrier sits mostly in the car's wake and costs much less, often a few percent at most.
• Empty roof bars. They still add drag. Take them off when you won't need them for a while.
• Open windows. At highway speeds, open windows add drag. Around town the effect is small.

Weight

Every pound you carry has to be speeded up and lifted up hills. US guidance puts the cost at about 1 percent of mileage for each extra 100 pounds, a little more in small cars. That's minor compared with speed, but heavy items left in the trunk for months add up. Clear out tools, sports gear and bags of salt you don't need.

Trip length and engine temperature

An engine is least efficient when it's cold. It runs a richer fuel mixture until it warms up, and the oil is thicker. A two-mile trip may end before the engine reaches normal temperature, so the whole trip runs at poor efficiency. Combining errands into one longer trip, starting with the farthest stop, lets the engine warm up once and stay warm. The weather and seasons guide covers the winter version of this problem.

Engine load and accessories

Air conditioning is the largest accessory load. In very hot weather it can cut mileage sharply, especially on short trips. Rear defrosters, seat heaters and powerful sound systems draw electricity that the engine has to make. None of these are reasons to be uncomfortable or unsafe, but switching things off when you don't need them helps a little.

Terrain and traffic

Hills cost fuel on the way up, and you get only part of it back on the way down. Stop-and-go traffic is the worst case for a conventional car, because each stop throws away the energy of the last start. Hybrids and electric cars recover some of that energy through regenerative braking, which is why many hybrids have a higher city rating than highway rating.

Fuel type

Use the fuel grade your owner's manual calls for. Premium fuel gives no mileage benefit in a car designed for regular. Fuel blends with more ethanol hold less energy per gallon: common E10 gasoline holds a few percent less than pure gasoline, and E85, used only in flex-fuel vehicles, holds a lot less, so you go noticeably fewer miles per gallon on it.

Fuel economy FAQ