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Tire Tips

FAQs that tell the how—and the why.

Here are the basics: Keeping your tires properly inflated by checking tire pressure at least once a month and following guidelines in your vehicle’s owner’s manual will extend their life and reduce fuel consumption. Once a month is also the recommended time to check tire wear—two for one!

Here are some frequently asked questions we hope provide some more insight. 

Just what is proper inflation, anyway?
It’s simply making sure your vehicle’s are at the pressure specified by the vehicle’s manufacturer. The right tire pressure varies by vehicle and by model. The information can usually be found on a sticker inside the driver’s door jam, in the glove compartment, or in the vehicle owner’s manual. Do not use the tire pressure listed on the tire itself—that’s the maximum pressure to which a tire can be inflated, not the pressure for optimum driving and efficiency.

When is the best time to check tire pressure?
The rule of thumb is to check tire pressure when your vehicle has been driven less than a mile or has not been driven for at least three hours, preferably overnight. If you don’t want to account for temperature, check when the temperature is about 70 degrees Fahrenheit. Tires can lose a pound per square inch of pressure each month, so be sure to check monthly.

OK, so what about temperature? Do I have to carry a thermometer, too?
That’s actually a good question if you want to keep your tires healthy. Tire pressure can drop one-half to one pound per square inch (psi) for every 10-degree drop in temperature. Compensate by increasing tire pressure accordingly. For example, a tire whose optimum pressure is 30 psi at 70 degrees Fahrenheit would be inflated to 34 psi at 30 degrees.

Can’t I tell if a tire is underinflated just by looking at it?
Not any more. Today’s high-tech tires have to be “low” by at least 50 percent before you can tell just by looking. The good news is that checking tire pressure is easy. Just use a tire gauge at least once a month. 

What happens if a tire is underinflated?
In engineering terms, underinflation reduces a tire’s effectiveness by preventing it from operating within its design specifications. That means underinflated tires . . .

• Increase tire deflection, which increases heat buildup and “rolling resistance.” Tire deflection is the flexing  of the tire tread and sidewalls where the tread touches the road.
• Reduce mileage by up to 5 percent
• Reduce tread life by as much as 25 percent—or about 10 percent for every 10 percent of under inflation.
• Weaken tire structure resulting in blow-outs or other failures.
• Reduce steering responsiveness and stability while cornering.
• “Break traction” more easily, which can affect handling.
• Affect weight distribution and in turn steering, alignment, braking and may cause your vehicle to “pull” one way or the other.

Can I tell by driving if my tires are at the right pressure?
Generally not, unless either you’re extremely sensitive or one of your tires is extremely low. However, you can get a good idea of what underinflated tires do to your vehicle’s efficiency level by riding a bike with an underinflated tire. You’ll notice increased rolling resistance because your legs work a lot harder. And you can see just how much tire deflection increases—that is, how much more rubber meets the road. You might even have trouble steering or cornering.

Aren’t overinflated  tires supposed to be good for driving, if a little bumpy?
The answer here is “It depends.” Serious racing drivers might overinflate their tires because that reduces rolling resistance a bit and can improve stability while cornering. But the average driver should not follow their adventurous example, even if a bumpy ride seems a small price to pay. Overinflated tires . . .

• Are damaged more easily by potholes
• Lead to irregular tread wear.
• Increase internal noise levels.
• Can mean a rougher ride
• Decrease deflection.  Less tire surface is in contact with the road and that can reduce control, especially in snowy or rainy weather.

Is nitrogen really better than air in your tires?
An interesting factoid for office cooler conversation: Air is already 78 percent nitrogen; only 21 percent is oxygen.

According to the National Highway Traffic Safety Administration (NHTSA), "More than a quarter of automobiles and about a third of light trucks (including sport utility vehicles, vans and pickup trucks) on the roadways of the United States have one or more tires underinflated 8 pounds per square inch (psi) or more below the level recommended by the vehicle manufacturer."

So the potential savings could be great if nitrogen performs as claimed. Fortunately for us, Consumer Reports did a yearlong test in 2006-2007. They took identical tires and filled them to 30 psi. The air-filled tires lost an average of 3.5 psi. The nitrogen-filled tires did a little better, but still lost 2.2 psi.

An NHSTA test of hundreds of tires found the same thing, that nitrogen filled tires lost approximately two-thirds as much pressure as air filled ones. They also found that tires filled with nitrogen had the same rolling resistance as tires filled with air.

So any increase in mpg would come solely from proper inflation.

How do I know when to replace a tire?
Good question. During your monthly tire pressure check, also take a look at tire tread wear. Here are two ways to do that.

• Turn a U.S. penny upside down and slide it into a tire’s tread. If you can see all of Abraham Lincoln’s head, it’s time for new tires. The reason is that the tread is 2/32 of an inch deep or less. In many states, you’re breaking the law if you drive on such worn tires. It’s important to test in several spots about 15 inches apart because the tires could be wearing unevenly.
• “Wear bars” built into tires also indicate when time’s up. If they are flush with the outside of the tread, it’s too late just to AIR-up. You’ll need to buy new tires. 

Are other people’s tires in good shape or do I need to drive even more defensively?
In 2009, thanks to the Rubber Manufacturers Association, tire retailers checked some 7,000 vehicles for tread wear in 12 states.  Here are the deflating results:

  9%     Vehicles with four properly inflated tires.

11%     Vehicles with at least one bald tire.
49%     Vehicles with at least one underinflated tire.
64%     Drivers who don’t know how to check tread depth.
85%     Drivers who do not check tire pressure properly.



Sources: www.fueleconomy.gov; www.edmunds.com; The Rubber Manufacturers Association
www.rma.org; Bridgestone Firestone www.tiresafety.com; National Highway Traffic Safety
Administration www.nhtsa.dot.gov; www.foxbusiness.com; www.tirerack.com