Tires: Passenger Vehicles and Light Trucks

Used and New Automobile Tires

What are all of these numbers?!?  Tire specs are standardized by the Tire and Rim Association.  The purpose of the Tire and Rim Association, Inc., includes the establishment of interchangeability standards for tires, rims and allied parts.  There are many specifications to a tire including max load, tread, max pressure, rim width, section width, tread width, overall diameter, and revolutions per mile.  But we aren't going to discuss them all.  What do you really need to know to buy a tire? The tread depth and the tire size measured by width / ratio / diameter.

How Do I Know What Tire Size I need?

Look in the owner's manual for your vehicle or on the metal tag often found inside the driver's door, glove compartment door or fuel hatch.  Otherwise look on the sidewall of your existing tires.

"P" designates passenger or performance tires.  "LT" designates light-trick tires.  The example below shows a Passenger tire, 185 width (millimeters), 75 aspect ratio (height/width), R (radial construction), 14 diameter (inches).  Towards the end you have 82 load rating and an "S" speed rating.  To make things simpler, just match the specs for your vehicle.

Understanding Key Features of a Tire

1. Temperature ratings - The letter signifies how well a tire resists heat: A is best, B is intermediate, C is acceptable.
2. Speed ratings - The letter designates the maximum speed in miles per hour: M is 81, N is 87, P is 93, Q is 100, R is 106, S is 112, T is 118, H is 130, V is 149, W is 168, and Y is 186.
3. Traction - Indicates how well a tire can grip the road surface, rated as AA, A, B, or C.  AA-rated tires offer the maximum grip!

But, your entire purchase is most affected by the tire's remaining depth!  If it is almost worn out then what is the point of buying it?

What is Tire Depth?

Tread depth is a measurement between the top of the tread rubber (surface of tire) to the bottom of the tire's deepest grooves.  In the US, tread depth is measured in 32nds of an inch.

* Tread depth is best measured with a tire tread depth gauge , or see your car's local dealership to measure it for you.

New Tires typically start with 10/32" to 11/32" of original tread depth, while dedicated winter tires and light truck tires typically are deeper.

When is a Tire Worn Out?

Tires are legally "worn out" in most States when they reach 2/32" of remaining tread depth.  For example, a tire that starts with 10/32" of original tread has only 8/32" of useable tread!  So, subtract 2/32" from the tires you are about to purchase and you will have calculated the legal usable tread depth.

Another way to put it is a tire that started with 10/32" that is being sold on eBay with 6/32" of remaining tread is 50% worn.

The example is calculated as 10/32" - 2/32" = 8/32" of usable tread.  Take the remaining tread of 6/32", as advertised, and subtract 2/32" to get 4/32" tread.  Since this is 50% of the original usable 8/32", the tire is 50% worn.

* Tire Wear Formula: 8/32" / (Advertised Remaining Tread - 2/32")

Who Will Install it?

Alright, so you know what tires you need, found them on eBay, and are ready to buy.  What about installation?  Walmart Tire & Lube automotive will change your tires for under $9.76 per tire at the time I wrote this, up to a 17" tires.  If you purchased larger tires, seek out a quote from local mechanic because they do not have the equipment to do it.

The following are a few tire specifications for picky people and tire geeks, and those with stronger feelings for their tires:

Maximum Load

A tire's maximum load is the most weight the tire is designed to carry.  Since a tire's load carrying capacity is relative to the tire's size and how much inflation pressure is actually used, maximum loads are rated with the tire inflated to an industry standard.  Maximum load is used to separate tires that share the same physical size, but differ in strength due to their internal construction.  The "higher" the load, the more air pressure it can hold, which allows it to hold more weight.

Maximum Inflation Pressure

A tire's maximum inflation pressure is the highest "cold" inflation pressure that the tire is designed to contain.  While a passenger car tire's standard load is rated at 35 psi, many performance and touring tires are designed to contain up to 44 psi (and are branded on their sidewalls accordingly).  This is to accommodate any unique handling, high speed, etc.  The tire's maximum inflation pressure is indicated in relatively small-sized print branded near the tire's bead (adjacent to the wheel).

I have rambled long enough.  Those that need additional tire information before purchasing a set of new ties on eBay, please seek medical attention.

Happy tire hunting.