Congratulations! You just purchased your very first car. You followed all my advice in my Factoid http://factoidz.com/a-guide-to-buying-a-used-car/ and got a really good car for the money you had to spend. Now, with Spring Break right around the corner, you want to get your pride and joy ready for your first road trip with your friends. You’ve noticed that the thread on the front, driver’s side tire is really bad and you know that the tire needs to be replaced. What you don’t know, being more of the literary type than the car type is whether you can replace just that one tire, whether you need to replace both front tires, or whether you should replace all four tires at the same time. Hopefully you will be able to answer those questions for yourself after reading this Factoid.
What do all the numbers and letters mean
Look at the sidewall of any tire and you will see a string of numbers and letters similar to this TIRE NAME P215/65R15 95H M+S MANUFACTURER. What do these numbers and letters mean? What do they tell you? TIRE NAME is self-explanatory, as is the tire manufacturer’s name i.e. Firestone. What does the P215/65R15 tell you. The “P” indicates that the tire is a passenger car tire. The “215” indicates the nominal width of the tire in millimeters. The “65” is the aspect ratio of the tire, the ratio of tire height to tire width. The “R” indicates that it’s a Radial tire. The “15” is rim diameter code. The “95H” is the load index and speed symbols. The M+S indicates that the tire was designed for severe snow conditions. There are also a bunch of other numbers on the side of the tire that give the maximum permissible inflation pressure; thread ware, traction, and temperature grades; maximum load rating; DOT registration numbers; and tire ply composition and materials used. What do these numbers man and are they important to you as a driver?
Tire size
The tire size i.e. P215/65R15 is the single most important piece of information for the consumer. Tire size includes tire width, the aspect ratio, tire type, and the rim diameter. Get any of these numbers wrong and the tires may not even fit the rims. A lot of bad things can happen when you make changes in this area so don’t experiment in this area, leave the experimentations to the experts.
Speed and Load Rating
Should we be concerned with the load and speed rating i.e. “95H”. The speed rating has little meaning for the everyday driver because there aren’t any tires that will blow out at 70 MPH but there are many tires that will blow out at 120 MPH so unless you plan to drive on the Autobahn you have no need to be concerned about the speed rating. The Load Rating isn’t of much interest to the average driver either. The Load Rating is only important if you are buying tires for a truck and you are planning on hauling some serious tonnage.
Thread ware and Traction
These are of concern to every driver. The higher the thread ware rating will last longer than tires with a lower tire thread ware rating but there is a trade off between handling and tire life. If cornering and handling is important to you, if you like to put the hammer down on winding mountain roads, then go for a tire with a high dry traction rating and a lower thread ware rating.
Uniform Tire Quality Grading (UTQG)
Uniform Tire Quality Grading is a US government mandated tire rating system. It is required for all Passenger type tires, except those used in a light truck application, and those classified as “mud and snow” tires. For best performance all the tires on your car should have the same UTQG.
Replacing tires on a front wheel drive vehicle
Unless both front tires were relatively new and you have to replace one because it was damaged beyond repair, it’s never a good idea to replace only one tire. When there is a measurable difference in thread depth and thread wear pattern dangerous handling conditions can arise.
Do I need to replace all four tires at the same time?
No, not unless you are switching from all-weather tires to snow-tires on a front wheel drive car. On a front wheel drive car if the front wheel have traction going into a turn and the rear wheels don’t, severe loss of control can result.
In conclusion
Buy the best quality tires that you can afford.
