A metric tire size of 235/70HR16 is equivalent to an ISO metric size of 235/70R16 109H and may be found on some Explorers, Grand Cherokees, and Discovery Series I sport/utes. In both cases, the "R" stands for radial, and the "H" is the speed rating (more later). The "109" in the ISO metric system indicates the load index for the tire, and the "109H" speed and load combination is often called the "service description." Since the ISO system is gaining in popularity, we'll use that example for explanation.

The first number in the 235/70R16 109H tire listed above, 235, is the section width of the tire, given in millimeters. This doesn't mean the tread is that wide, but rather that the widest part of the tire, generally halfway up the sidewall, is about 235 mm across. The second number, 70, is the aspect ratio, also known as profile, which is the height of the sidewall expressed as a percentage of section width. On this tire, the sidewall is 70 percent of section width, or about 165 mm high (on tires with no aspect ratio listed, such as 205R16, the aspect ratio is assumed to be between 80 and 82). From above, R is radial, though few new vehicles come with anything else. The third number, 16, is the wheel diameter, and the only value given in inches. Except for Michelin TRXs of 20-plus years ago, all diameters are in whole or half inches (15, 16, 16.5, 17, 18, etc.). The load index number, 109, refers to the weight the tire can carry, in this case, 2271 lb.

However, the load index is not a linear function (refer to the chart for common values). A load index of 120, which might be found on a 3/4-ton HD pickup, carries twice the weight of a load-index 95 tire. Also note load ratings will vary for tires that may be used on dual-wheel applications because of heat buildup, so a tire rated for 3042 lb as a single will rate only 2778 lb run as duals (and its service description would be listed with two numbers, e.g. 119/116S). Finally, the last letter speed rating (see chart), a value determined by laboratory testing, indicates the speed the tire can run continuously (actually meaning at least a half an hour) without failure. Note that speed rating does not necessarily correlate with alphabet position, and there are speed ratings not listed in the chart, though few light trucks exceed 149 mph.

The flotation sizing system is derived from the obsolete numeric system, with all dimensions given in inches. Using a Jeep Wrangler's optional 30x9.50R15LT C as an example, the "30" refers to the height of the tire, "9.50" is the section width, "R" for radial, "15" is the wheel diameter, "LT" for light truck, and "C" is the load range. Load ranges used on LT tires increases weight capacity with alphabetical order: A range of "B" or "C" is common for compact and midsize utilities and 1/2-ton pickups, with "E" common for heavy-duty pickups (many dualies use "D"), and "G" and "H" are suitable for motorhomes and commercial trucks. The weight limit, in pounds and/or kilograms, is listed on all flotation-tire sidewalls.

All tire load-ratings are given at a maximum cold pressure listed on the sidewall. A common mistake is to assume it's the pressure the tire should be run at. The recommended pressure for your vehicle will be found on a placard on the door or post, fuel door, glovebox, or owner's manual (see air-pressure sidebar for more).

An "M&S" on the sidewall means the tire is mud and snow capable. No Truck Trend tester will drive into deep mud just because the tire says M&S, though law-enforcement agencies will frequently look specifically for that designation when chains are required, unless you have snow tires. It should be noted that tires designed specifically for mud or snow are not designed for both and will have obvious tread differences.