Underinflated tires reduce gas mileage and causes tires to prematurely wear out. Many years ago tires were inflated to approximately 32 psi, today most tires are inflated to 40 psi. A simple, inexpensive tire gauge is used to measure tire pressure. If you don’t know how to use a tire pressure gauge, most repair shops or an automotive parts store will show you how to use one.
The size of your tire is printed on the tire and it starts with a “P.” After the “P” are two numbers an “R” then two more numbers. For example a common tire we replace is a P215/65R15. The “P” means passenger tires, the 215 is the width of the tire in millimeters, 65 is the ratio of the height to the width of the tire (basically the distance from the edge of the rim to the face of the tire), and 15 is the radius of the rim in inches.
Load Index & Speed Rating:
After the size is a group of two to three numbers and a letter. The numbers are a “Load Index” and indicate how much weight the tires can support. A load index of 94 supports up to 1477 pounds. The letter after the load index is the speed rating- the maximum safe speed for the tires. A “T” speed rating is safe up to 118 miles per hour. A “W” speed rating is safe up to 168 miles per hour. Between T and W there are many other speed ratings. The higher the speed rating the better the tire ‘grips’ the road. However, since it grips the road better than a lower speed rated tire, the higher speed rated tire wears out quicker.
You may use a higher speed rated tire than the manufacturer recommends, but you should never use a lower speed rated tire.
Winter- Summer Tires
I remember as a kid helping my father change his tires in the fall from summer tires to winter tires, and in the spring we’d change them back. I don’t know how the tire manufactures did it, but we no longer have to change tires for the winter and the summer. There are winter tires, but you probably don’t need them. If you look at your tires and see “M + S” printed after the speed rating then your tires are good for winter and summer.
Tire Manufacturing Date Stamps:
Starting in 2000 tires have a manufactured date stamped on the side of the tires. Sometimes the date is on the inside of the tire, and sometimes it is on the outside of the tire. It is a four digit code. The firs two digits indicate the week the tire was made and the last two digits indicate the year it was made. So a ‘4010’ would mean the tire was made in the 40th week of 2010.
TPMS (Something new):
A few years ago a rash of roll-overs was attributed to old and underinflated tires. After investigating why these vehicles rolled-over Ford recommended that tires be replaced every five years regardless of wear. The NTSB mandated the use of a “Tire Pressure Monitoring System.” This system is referred to as “TPMS.” All cars and trucks manufactured since 2010 have this system.
TPMS use a wireless pressure sensor in the tire to communicate with an onboard computer. If a tire is underinflated by 20% or more a light on the dash illuminates indicating an underinflated tire. The sensor in the tire is battery operated and the valve stem is part of this system. When the battery goes bad, or the valve stem breaks the sensor must be replaced, and they are not cheap. While it’s too early to tell, I have been told the battery will last for seven years.
On some cars, after the tires are rotated, the TPMS computer must be reset or the dash light will come on falsely indicating an underinflated tire.
While TPMS have added a measure of safety, they come with a cost.