What do the numbers on the tire sidewall meanHow To Improve Wheels and Tires? Molded into the sidewall of every tire is a series of letters and numbers that tell you everything you need to know about that particular tire if you know how to decipher the code.

Initially, tire makers stamped their tires with inch markings (many still do), but metrification has taken over now, so the majority of markings are given in metric sizes. Typical markings may look like this: 4.00H-18 4PR (inch) or 100/90H-18 (metric) or 100/90R18 73 H (metric radial).

The preceding nomenclature describes the tire’s basic dimensions, speed rating, and whether it is bias-ply or radial. You may also find some tires marked with an alphabetical-numerical code (MM 90-18 Load range B, for example).

Tires marked with alphanumeric designations are the least popular, and tires so marked aren’t as common, but you do see them. By the way, in case you hadn’t suspected, all four tires listed in this paragraph are roughly the same size.

Here’s how to decode inch tire sizes, using 4.00 H-18 4PR as our example:

• 4.00 is the tire width in inches.

• H is the speed rating, indicating that this tire is rated for sustained speeds of 130 mph.

• 18 is the rim diameter in inches.

• 4PR indicates the number of plies, four in this case.

Here’s how to decode metric tire sizes, using 100/90H-18 as our example:

• 100 is the tire width in millimeters.

• 90 is the aspect ratio, 90 percent, indicating a height of 90mm.

• H is the speed rating, 130 mph in this case.

• 18 is the rim diameter in inches.

The code stamped into a radial tire’s sidewall is slightly more complex, but just as easy to decipher.

Let’s break down a 170/60 R 18 73 H.

• 170 means the tire has a width of 170mm.

• 60 is the aspect ratio; in this case, it’s 60 percent of 170, or 102mm, meaning this tire is 102mm high.

• R means this tire is a radial.

• 18 is the rim diameter in inches.

• 73 is a code that designates the maximum load that the tire can bear. It doesn’t mean much unless you have a load chart to tell you that a tire rated at 73 can support a load of 805 pounds.

• H is the speed rating, 130mph in this case. Although you may not encounter many, let’s take a look at the last category of alphanumeric tires using the example of MM90 18 Load range B.

• M stands for the motorcycle.

• M means the width is 4 inches.

• 90 is the aspect ratio, in this case, it’s 90 percent of 4 inches, meaning this tire is 3.60 inches high.

• 18 is the rim diameter in inches.

• Load range B; this is a curveball. Load ranges are listed alphabetically as well as numerically, A being the lightest.

Tire manufacturers are free to use any marking style and to add any other information they want. Other sidewall stampings will indicate whether the tire is intended to be used with a tube or without and the maximum tire pressure.

If the tire is directional (meaning that it is intended to be mounted so it rotates in a given direction) there will be arrows on the sidewall telling you exactly what direction the tire is supposed to rotate. The arrows are generally marked front and rear. The tires should always be mounted so that when the arrow is at the top of the wheel it points forward when the tire rolls forward.