“I have to go into the shop to get tires again,” my friend shook his head sadly and pointed at his car. I asked, “Didn’t you put these on last spring?” He nodded, “Yeah, they keep going bald on me.” I examined the rear tires and found though they had good tread on the edges, the centers were nearly bald. I asked, “What tire pressure are you running?” He squinted, “45 psi, to get better fuel mileage.”

I thought about how to break the bad news to my friend. Not only was he spending hundreds of extra dollars burning through tires, but he probably wasn’t even getting better fuel mileage by over-inflating his tires. But before you laugh at him, know that this is an incredibly common myth.

The myth is this: disregard your manufacturer’s recommended tire psi (printed on the sticker in your door sill), and over-inflate them by about 10 pounds. Sure, you won’t get the same traction. But in exchange, you’ll enjoy improved fuel mileage.

I will admit, this rumor has some basis in fact. If you let your tire psi get too low, you’ll see your fuel mileage decrease. Under-inflated tires can cost you about 0.4% of your mpg for every 1 psi they fall below your manufacturer’s recommended range. So if your manufacturer recommends 35 psi, but you let them get down to 25 psi, you might see 4% fewer mpg.

Why do low tires cost you mpg? Because it increases your car’s rolling resistance. The low tires flex and more of the tread touches the ground. This is great for traction. But it means your engine must work harder to get your car rolling and to keep it rolling.

My friend hoped that by increasing his tire pressure to 45 psi, so little of the tread would touch the ground that the reduced rolling resistance would translate to fuel savings. And on the nose, this assumption makes some sense. But it’s been disproven.

Popular Mechanics tested this myth by doing a 400 mile drive twice, in the same car. First, they ran it at 45 psi (just 5 pounds below the tires’ 50 psi maximum). Then they ran it again at 32 psi. The result was…(drumroll please) the exact same mpg.

The truth is that manufacturers want the best possible mpg. So your recommended tire pressure is often optimized for exactly that. Going any higher is unlikely to help you out. And, as my friend learned, only allowing the center of the tire tread to touch the ground will wear it out much faster.

Next, learn how to properly check and fill your tires in the video embedded below: