An underinflated tire is a problem.
Think about what it’s like to ride a bicycle with tires that are low on air — it’s like you’re trying to ride through wet sand, due to the excessive rolling resistance of those tires. Well, that’s what’s happening with your car, too. Every pound of air pressure (pounds per square inch, or psi) that your tires are low will cost you about .2% in fuel economy.
In addition, that added rolling resistance means heat buildup from excessive friction, and heat is the enemy of your tires. A tire that’s low by 5 or 6 psi can actually be at risk of failure as the steel belts heat up, the cords start to separate and the tire’s internal structure comes apart. Underinflated tires can also affect braking, traction, cornering and road manners; if you notice a persistent pull to one side, the first thing you should check is the air pressure of the front tire on that side, as a low tire can drag your vehicle in that direction.
How TPMS Works
Starting in the mid-2000s, the National Highway Traffic Safety Administration made TPMS mandatory on all new vehicles, as a result of the well-publicized Ford Explorer/Firestone tire failure incidents of the late 90s. Most TPMS systems use what’s called indirect monitoring:
- The rim has a small sensor mounted on its inside, at the edge
- As a tire loses air, its diameter changes and its rotational speed increases
- The TPMS sensor on the rim monitors rotational speed and transmits information to a processor wirelessly
- The processor then illuminates the TPMS light when the sensor indicates that rotational speed has changed past a certain threshold
Other systems use direct monitoring, which involves a sensor that’s mounted directly in the valve stem and can give real-time readings on the actual tire pressure (some can also monitor tire heat). The unit then sends wireless information to a processor, like in an indirect TPMS system.
Of course, these systems have to be reliable and have to be able to operate in a pretty hostile environment, with lots of dirt, all kinds of temperatures, exposure to chemicals and lots of other factors. While TPMS sensors are designed for long service, they should be checked whenever you have the tires rotated, and certainly should be changed when you get a new set of tires. When the TPMS is illuminated, it won’t let you know what tire is actually low; it’s up to you to get out with a tire pressure gauge and check for yourself.
TPMS Systems and Temperature
Here’s where things get a little dicey. You’ve probably heard that you should always check your tire pressure only when tires are cold, which makes sense since the air in your tires is a gas, after all, and a gas will contract when it’s cold and expand when it’s hot.
Your tires will actually lose one psi of pressure for every ten-degree drop in temperature; on a cold morning, it might be enough of a change in pressure to illuminate the TPMS light, and then the light might go out again once the tires are warmed up a bit from driving. If the TPMS light comes on and stays on, you’ll want to go ahead and check your tire pressure when you get a chance.
Bear in mind also that your tires will naturally lose about one psi of pressure per month, either around the seal at the bead or just from migration of air molecules through slightly-porous rubber. With that in mind, it’s a good idea to check the air pressure on all your tires once a month or even every time you top off the tank.
Finding the Right Tire Pressure Gauge
A tire pressure gauge should be one of the things you keep in your console, door pocket or glovebox. There are a few styles on the market — the time-tested pencil style, the dial-type and even digital ones — and truthfully, they all deliver about the same kind of accuracy. If you go with any of those, you’re doing the right thing…about the only thing we’d advise against, though, is trusting the tire pressure gauge that’s built into the hose at the gas station, since those can be inaccurate by as much as ten psi over or under the actual reading. Don’t have a tire gauge? Email me and I will send you one for free! Tire companies are always giving them away.