Beyond Tire Size: Shopping for New Tires Part Two
Last week I talked about tire sizing and helped you figure just what all the numbers mean when shopping for tires. Today I’m digging deeper into the alpha-numerical alphabet soup on marked on the wall of the tires.
Tires being one of the largest contributors to the safety of a vehicle, and the only thing that touches the ground, there are a bunch of rules and regulations that manufacturers need to meet in order to sell them in this country.
DOT number – On one side of every tire is a number preceded by DOT then 1B3D5F7H9J1 1211. The last four-digits are the week and year of manufacture, and any other numbers are the tires batch number, manufacturing location, brand and size.
If you are a real tire nerd, it is possible to decode it and know that two tires with different brands are, in fact, produced in the same factory.
Tire Age – The last 4 digits are the week of the year (12th in this example) and the year (2011 in my example).
Most shops have a quick turnover of inventory, but if you’re getting tires mounted at your local big box warehouse store, it’s not a bad idea to check the date. They buy in quantity to pass the saving on to you.
Tire tread these days lasts so long that often you need to replace them because of age even before they are worn bald.
Other numbers on there are also important–like the speed rating, load rating and the Uniform Tire Quality Guide (UTQG) treadwear rating, traction and temperature ratings. The UTQG was created by the DOT and the National Highway Traffic Safety Administration (NHTSA) to provide the consumer with information to compare tires. They aren’t perfect, but they do provide useful information.
The load and speed rating are the numbers and letters after the size. The size I talked to death last week, but it looks like this “225/60P16″ and then two numbers and a letter like “88T”.
Load Rating – Commonly from about 70 (737lbs) to 110 (which equals 2337lbs) is the load rating. Remember if the car/truck, cargo and passengers are divided evenly among the four wheels, a set of 70 rated tires can carry 2948 lbs. But nothing is ever perfectly balanced, and then there are braking and acceleration forces and cornering. The best thing to do is find out the original load rating and never use a tire rated below that.
Speed Rating – Years ago, only performance tires were speed rated, starting at “S” (112mph) and going up to “Z” (149mph+). Then crazy exotic cars were developed with top speeds way above 149mph. And then not so crazy cars like BMW M3s and Cadillac CTS-Vs followed that could go more than 150mph with four passengers in them and the air conditioner on. So speed rating still start at “S” and go T, U, H, V, Z with “W” and “Y” above those.
Treadwear – Usually a three-digit number. Tires are run over the same 7,200 mile road and compared to a control tire. The number indicated how it wears compared to the control, with a 200 lasting twice as long, a 300 lasting three times and so on. You can’t set your watch to it, but you can assume two different tires with 300 ratings wear similarly, and tires with a 400 will last even longer.
Traction – Letter grade from AA to C, with AA being the highest. It’s based on a test of wet surface traction in braking, and acceleration. Drive where it rains a lot or drive without snow tires in the winter and you should look for AA or A rated tires. Driving your Corvette convertible only on nice summer days? C rated tires most likely have the fewest grooves and the most dry traction on a sports car.
Temperature – The A, B or C temperature rating is actually an indication of sustained high speed ability. The lowest C rated tire can cruise at 85mph with no problems. An A tire is good for more than 115. So if you’re planning on making a run for the border or setting your own coast-to-coast speed record, you are going to need an A grade tire.
That’s pretty much a rundown of all the info available on a tire. Before walking into your big box warehouse store or local tire retailer, do a little online research. If you have a car popular with enthusiasts, do a quick search of the forums online and you’ll get tons of opinions.
Big online retailers like Tire Rack have years of test data and hundreds of customer comments about a huge number of different brands and models. Pick out a winner or two then make a few calls–or send out a few emails and see who’s got them locally and for how much with installation and all the extras. The worst thing to do is blow out one tire and decide to stop at the next tire shop you find and buy four new tires–and a new spare.
The U.S. DOT has not set a guideline for maximum useful tire life, but other countries have settled on ten years as a general rule. By the way, be sure to check your spare periodically for proper inflation. If you have a full-sized spare, get a new one after ten years too.