Jack Williams

Need Air? You Can't Tell by Looking.

If you learned everything you know about cars from dear old Dad, he may have told you that a quick visual inspection of your tires is usually sufficient to determine whether any of them need to be topped up with air. Just check for a bulge in the lower sidewall, where the tire meets the ground, right?

Wrong.

Modern car wheels have generally larger rim diameters than they did 40 years ago, and that means the sidewalls of your car's tires are narrower (or shorter, if you're looking at the part near the ground) than those that shoed your father's bachelor wagon.

That measurement of the tire is called the aspect ratio. It's a percentage of the tire's width across the tread. If your tire is 180 millimeters wide, and your sidewall is 90 millimeters wide, its aspect ratio is 50.

In a tire with a smaller aspect ratio of 40 to 65, the visual difference between 30 pounds per square inch (PSI) and 20 pounds per square inch is going to be too small for most people to notice while looking at their car in the driveway.

Dad probably also told you to keep a tire gauge in your car, and to use it to check your tire pressure. Follow that advice, and leave the visual tire inspections to your auto care professional.