Rick's Free Auto Repair Advice

What’s is the correct tire pressure for larger tires

What is the correct tire pressure

I see this question all the time on auto forums and I see lots of horrible advice. If you want to know what is the correct tire pressure, just look on the decal inside the driver’s door pillar or on the driver’s door. NEVER fill your tires to the maximum listed on the site of the tire. THAT’S DANGEROUS.

Even if you’ve installed a different size tire, follow the tire pressure on the decal. Why? 

Because the weight of your car didn’t change just because you changed to a larger tire. A larger tire needs more air volume, but it doesn’t need higher pressure. Weight of the vehicle and handling are what determines air pressure, not tire size.

Some people claim you get the best gas mileage that way. Let me explain why that’s dangerous and can actually cost you far more than any potential gas savings.

First, the decal pasted in the driver’s door area is put there by the manufacturer for a reason. The car maker knows the weight of the engine and transmission that was installed at the factory. And that tells them the weight of the vehicle. Tire pressure, regardless of what size tire you put on the car, is based on vehicle weight. Get it? A larger tire make take more air to fill, but once you reach the pressure listed on the decal, you should STOP filling. It’s the pressure that holds up the car, not the volume of air. And the amount of pressure needed is based on the weight of the vehicle and the handling characteristics designed into the suspension, not the size or design of the tire.

Now let’s talk about why it’s a bad idea to overfill. Once you go past the car makers recommended psi, the shoulders of the tire start to lift off the ground. You’re basically turning the tire into a doughnut. So you’re actually REDUCING the “contact patch,” that portion of the tire that’s in contact with the pavement. That means you have LESS traction, and LESS stopping power. It also means you’ll wear out the tire faster because all the wear is occurring on the center portion of the tread. Even more troubling is the fact that you’ve now dramatically reduced the tire’s ability to pump water away from the tread, increasing your chances for hydroplaning. The tread in the center of the tire pushes water towards the shoulders. But the shoulders are responsible for slinging the water out. The slinging action creates a mini vacuum, which actually sucks water away from the center tread. So by overinflating, you reduce the pumping ability of the entire tire.

Let’s move on to safety and wear. A tire is designed to handle a certain amount of road shock. Once road shock exceeds the tire’s ability to absorb it, the wheel moves the vehicle’s spring and shock. The goal of the shock is to reduce spring oscillations. When you overinflate a tire, you reduce its ability to absorb minor road shock. So more of the road shock is transferred to the springs, shocks and the control arm bushings. See where this is going? Yeah, more rapid wear on suspension components. And they’re not cheap. Let’s go back to shocks for a second. I said the shock’s job is to prevent spring oscillations. If a vehicle didn’t have shocks, the tire would bounce and keep bouncing. Every microsecond the tire is in a bounce is less time that it’s in contact with the pavement. That means less control over steering and less traction during braking. So a tire that’s overinflated can require longer braking distances.

Put this all together and you can see how any minor increase in MPG you get from over inflation is eliminated by the increased wear and tear on your suspension components, faster tread wear in the center area, the risk of hydroplaning, and increased stopping distances.

So the next time you see some wise guy recommend inflating to MAX pressure listed on the tire, ignore the advice. It’s WRONG.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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