There’s a popular theory among car maintenance DIYers that overinflating your tires will improve your fuel mileage during a long highway trip. Because low tire pressure decreases your fuel efficiency and higher psi leads to less rolling resistance, the theory may even seem true to you. There’s only one problem: increasing fuel mileage from overinflated tires is a complete myth.

Should you check your tires before a road trip?

Double-checking your tire pressure is a critical part of your road trip preparation. Every pound per square inch (1 psi) your tires are low will cost you a 0.4% drop in gas mileage. You can’t afford not to double-check your tire pressure before a long road trip.

Check the placard inside the driver’s side door of your car. This door tag lists important specifications such as the automobile manufacturer’s recommended tire size and tire psi.

Next, take your vehicle to a gas station with an air compressor to fill your tires. Set the compressor for your recommended psi and let it adjust your tires. Alternatively, you can buy your own tire pressure gauge. This way, you can check your tires before your trip and continue to monitor them during.

Does overinflating your tires improve your fuel mileage?

You may have heard that inflating your tires far past your manufacturer’s recommended psi will improve your fuel mileage. But this is actually a myth that could cause you to put yourself and your passengers in danger.

Underinflated tires flex more while you are driving. This means more of the tire’s rubber touches the road and this added traction improves braking and acceleration. For this reason, drag racers and serious off-roaders often “air down” their tires.

But at the same time, underinflated tires create more rolling resistance and damage fuel economy. It might seem true that overinflating your tires further decreases rolling resistance and thus improves fuel economy even more. But the truth is that automobile manufacturers’ recommended tire pressure is the best possible psi.

The experts at Popular Mechanics actually put this myth to a test. They did the same road trip twice with the same car. During one journey, PM inflated the tires to the recommended 32 psi. During the other trip, the publication ran all four tires at 45 psi.

What was the result? The vehicle got the exact same mileage. The overinflated tires may have returned a 0.05 mpg increase. But the cost could have been deadly.

Are overinflated tires dangerous?

The sidewall of your tire has a maximum recommended psi. This may be much higher than the vehicle manufacturer’s suggested pressure, and may be a tempting psi to run. But overinflating your tires this much leads to dangerous handling and an uncomfortable ride.

When Popular Mechanics tested fuel mileage by taking a long road trip in a car with its tires at 45 psi, it collected some sobering information. Firstly, any fuel mileage increase was between negligible and nonexistent. Secondly, the overinflation inhibited the car’s handling.

Because overinflated tires do not flex as they roll, much less of their rubber touches the road than their engineers intended. This negatively impacts acceleration as well as braking. But in addition, automakers intend tires to act as shock absorbers. Popular Mechanics reported that at 45 psi, its tires caused such a bumpy ride on the interstate, its garage-door opener fell off the sun visor where it had been clipped.

Overinflating tires does little for fuel mileage and negatively impacts the car’s dynamics. If there were a “free” way to get much better fuel mileage, don’t you think auto manufacturers seeking a competitive edge would have figured it out?

Next, read how to upgrade your tire-changing tools or see how to prepare your car for a road trip in the video below: