Most tread patterns are asymmetrical, symmetrical, or unidirectional. Asymmetrical tires used on SUVs and pickups have different patterns on the inner and outer tread edges, usually more aggressive blocks on the inside for loose surface traction and more rubber on the outside for pavement cornering grip. Unidirectional tires are designed for efficient removal of water at speed and are least popular for SUVs that go off-pavement. Symmetrical tread tires can be run in any direction, although they're sometimes marked "inner side" (or outer) for lettering or sidewall protection. Tread-wear indicators are the small bumps in the grooves that show when the tread has worn down to replacement (or illegal) tread depth.

Some data books list a tire's tread "void ratio"--the relationship between the tread's contact surface and the grooves and sipes cut into it. A void ratio of 40 percent, typical for an all-terrain tire, means 40 percent of the tread rubber is open groove or channel. Some people are surprised to find that a mud tire, especially one moderately worn, might actually outhandle an all-terrain tire on pavement because of the huge grooves in the tread. However, mud tires normally have big blocks of rubber, while all-terrains have smaller blocks and many sipes, so the mud tire actually has more rubber on the ground. The only deficit is that the large tread blocks sometimes deflect easier than small ones. It's not a rule, but large tread blocks usually generate more tire noise than less aggressive treads. And most tires today have staggered tread blocks, with progressively larger and smaller blocks alternating around the circumference to minimize noise. You can see this on your tires if you look closely.

Generalized conclusions are easy to find in tread designs. Wide circumferential grooves or channels will perform well in wet conditions. Big tread blocks will work better in mud or loose dirt and also tend to dig down into firmer snow or sub-surface clay and so forth. All-terrain or M&S tires with many small sipes tend to work well in some snow conditions, and all the small edges will grab small rocks and off-road obstacles better than expected. Tires with unbroken ribs (no blocks) around the circumference are designed for long miles and heavy loads and will be found on many 2WD heavy-duty pickups and the steer axle of commercial vehicles. Despite the fact that snow and rain are both versions of H20, snow tires don't always work well in mud, and M&S tires don't always work well in rain.

Sand brings different concerns to tire selection. On a light vehicle like a buggy, a big paddle tire that looks like a farm- implement tire will provide excellent propulsion. However, on anything with some weight in it, a paddle tire or aggressive off-road tire will often dig into the sand and bury you, while a smooth highway tread will better stay on top. Besides, most paddle tires are not highway legal.

What's for Me?
Picking the right tires can get overwhelming. However, as long as you evaluate your requirements honestly, it doesn't have to be. Sure, your kids and buds will love those 3-ft-tall mud tires, but do you want to put up with the noise? Are you really going to mud-bog your daily driver?

Frequent trips off-highway qualify for an all-terrain-type tire, provided you realize it won't handle pavement like a street tire. Even more time off-road, or off-highway in places that are normally wet, and a mud-terrain-style tire will be good.

Those who bought an SUV for winter traction and discovered that four-wheel drive is (A) not always that, or (B) that M&S tires aren't great snow tires, should check into tires like the Bridgestone Winter Dueler and Michelin 4x4 Alpine. These are dedicated SUV/light-truck snow tires, with literally thousands of small edges, and many people keep a set on a second set of wheels just for winter use.