Here's How Tire Pressure Actually Affects Gas Mileage

Every time you fill up your fuel tank, you may feel the sting of fuel prices and vow to drive with a lighter foot. But your driving habits may not be the reason you're losing out on gas, as underinflated tires are often to blame. Underinflated tires become compressed and sag under the vehicle's weight, which increases rolling resistance. This forces the engine to work harder than it would at the manufacturer's recommended tire pressure, and as the engine works harder, it burns more fuel, ultimately reducing efficiency. Driving with low pressure might not hurt your transmission or other important components, but it'll definitely place more strain on your car.

A study by Oak Ridge National Laboratory showed that tires with 25% less than the recommended pressure experienced a 2–3% drop in fuel economy. A 50% reduction in tire pressure resulted in a 5–10%. In the winter, you'll need to be especially mindful of your vehicle's tire pressure, as it drops further in colder temperatures, decreasing by one or two PSI for every 10-degree drop. This happens because, as air cools, it contracts and becomes denser, which reduces the pressure in your tires. 

Each vehicle has its own recommended tire pressure, which can be found on the driver's side door jamb and in the owner's manual. Never fill your tires to the pressure printed on the tire, as that number refers to the maximum pressure for that tire — not the recommended pressure. On top of reducing miles per gallon, an underinflated tire can also cause serious damage to the tire's structure, making it provide inadequate grip on roads.

On the flip side, you might wonder if overinflating your tires beyond the manufacturer's recommended limits will give you better mileage. Generally, though, tire manufacturers agree that the benefits are negligible. Overinflating tires will give a slight increase in fuel economy, but could prove to be potentially harmful. A test showed that a 40% increase in tire pressure above the recommended limit resulted in a small 0.12% increase in fuel economy. The gains are there, but they're minimal — and they could be dangerous. Overinflated tires can increase the risk of a tire failure, cause unstable handling, and worsen ride quality. Tire wear is also likely to increase, particularly in the center, when a vehicle has overinflated tires. 

Vehicle manufacturers take various factors into account when setting the correct tire pressure, including safety, tire life, fuel economy, and handling. As modern cars are equipped with systems designed to monitor tire pressure, it's easier to keep track of it yourself. Make sure you don't ignore the constant alerts from those systems, and that you inflate the tires to the correct pressure levels.