Question: What does a smooth suspension, 350-hp engine, sophisticated stability control, and massive brakes have in common?

Answer: They're only as good as the tires they ride on.

No other part of your vehicle connects you to earth, yet truck buyers look at engines, tow ratings, room, and gadgets, assuming proper tires are already part of the deal.

Cavemen probably didn't realize the ramifications of inventing the wheel. Progress has developed the simple concept into a more complicated object that's partially responsible for every aspect of vehicle dynamics, ride, safety, traction, and economy. The following primer isn't meant to make you an expert, but rather to equip you for coherent conversation with your tire specialist, because what you don't know could literally, well, you know.

Sidewall Hieroglyphics
The first pneumatic tires on an 1896 production car were simple things unencumbered by heat requirements because nothing went faster than a brisk bicyclist. Modern tires and the machinery they run on is more complex, so a wealth of information is printed on every street-legal tire sidewall. Certainly, many people look for the name and outlined white letters, but that's the least-important writing there. From the alpha numerics on a tire, you can tell what size it is, how much weight it'll carry, up to what speed, what it's made of, where, when, and what the government thinks of it.

Virtually every tire size on the market today may be expressed as metric, ISO metric, or flotation (inches). Metric and ISO metric differ mainly in where the load rating is listed. A P-metric designator is the same as ISO metric except that a P (for passenger car) precedes the size. LT (light truck, LT265/75R16E, for example) tires are similar, except that the load rating is a letter, just as it appears on flotation tires.

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.

Further sidewall information includes the Uniform Tire Quality Grading system, which is overseen by the U.S government. The UTQG lists three factors:

Treadwear: The projected life of the tire. From a baseline 100, a larger number means the tire is expected to last longer than average; a smaller number means faster treadwear.

Traction: A (best), B, or C, and is based solely on the tire's wet-road straight-line stopping distance from 40 mph and is no reflection of its propulsion or cornering ability.

Temperature: A (best), B, or C, which rates to what degree the tire dissipates heat. Naturally, an A-graded tire can run hotter, but with proper loading, pressure, and operation, this grade is almost irrelevant.

Finally, the sidewall also gives construction (number of tread and body plies and their materials), and whether the tire is tube-type or tubeless. The stamped DOT serial number can be used for identification if a tire is recalled and to determine where and when the tire was made.


SPEED RATING CHART
Symbol Speed, mph
M 81
Q 99
S 112
T 118
U 124
H 130
V 149


LOAD INDEX
L.I.# Load, lb
81 1019
85 1135
91 1356
97 1609
103 1929
109 2271
116 2756
120 3086
123 3417

Tire Construction
While Charles Goodyear is generally credited for the vulcanizing process (which makes rubber more flexible), England's Robert Thomson developed the pneumatic-tire concept. In elementary terms, a tire is no more than a group of materials that contain an air cushion around a wheel. These materials may include rubber, synthetic fabrics (rayon, nylon, polyester), and steel, and are wrapped in layers according to different designs.

Such designs have included belted, bias-belted, bias ply, diagonal ply, and radial. Since radials provide better ride characteristics and lower rolling resistance, they're the design of choice for standard equipment today. Michelin debuted a steel-belted radial in 1948, but it didn't become popular until the mid-'70s. No coincidence there, since that era marked the first gas crunch and softest-riding barges.

If you took a hacksaw to your tire, the cutaway would show the tread, belts, and beads. The belts stabilize the tire carcass and maintain the integrity of the air cushion by keeping out debris and sharp objects. Each of these belts, or plies, does a specific job, which is why the number and type vary by tire and tread or sidewall. The way these belts are wrapped around the tire defines radial (or belted, bias-ply, etc.).

The tire is held on the rim by the beads, the round cross-section inside edge of the tire. Inside the bead is the anchor cable, which acts much like the main cable on a suspension bridge and holds all the tire plies together. This cable is protected by a hard rubber cover that helps keep the sidewall stiff next to the rim and often by a layer of fabric to save the tire from rim abrasion or scuffing during installation. Wheel-mounted bead locks physically lock the tire on the wheel instead of relying on air pressure. Bead locks are not used on production vehicles, but in situations where the tire may pop off the bead because of low air pressure (sand, off-road racing) or where the wheel may spin inside the tire because of low pressure and/or excessive power (dragsters, sand buggies).

The tire's top layer is the tread, and with the exception of curb guards--thick ridges at the widest point of the sidewall--the only part of the tire designed to contact the ground. The tread does wrap around the side of the tire slightly for cornering grip and better water evacuation, but good off-road tires have extra tread or sidewall protection.

Tread Options
Apart from a data book or the sidewall, the tread pattern will tell you more about a tire than anything else. The contact patch (the area where tire meets ground) consists of a series of grooves (channels) that serve as primary water passages, ribs that run around the circumference of the tread, and individual blocks of tread rubber that often become much larger blocks after the tire has worn down. Sipes are the small edges in the tread blocks and ribs.

The basis for tread material is rubber, but there could be 30 or more different materials used in the compound. If a compound is called "hydrophilic" it means it's attracted to water, which makes it an excellent material for dedicated snow or rain tires. However, hydrophilic compounds tend to wear faster, just like sticky tire compounds. Recently, Goodyear introduced a compound derived from cornstarch and mixed with synthetic rubber, called BioTred. This compound is said to save fuel (and therefore lower emissions) and give better wet-braking performance, but sizes are limited because it's a new technology.

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.

Heavy-pickup drivers often do what truckers do, running a traction tire on the drive axle and a highway tire on the front steer axle. This often works on 4WD versions as well, because aggressive traction tires tend to wear out quickly on heavily loaded steer axles. Also remember that a camper added to any independent front-suspension pickup will probably affect front-wheel alignment, and a highway ribbed tire will put up with misalignment a little better than a traction tire.

If you're being honest and are pavement-bound for most of the year, a street-oriented tire is your best bet. Many manufacturers are building tires "designed for SUVs" these days, so the choice arena is large. You can also opt for a "Plus" conversion for higher performance at some sacrifice.

Ford's Harley-Davidson F-150 comes with a Plus conversion built in, with 20-in. wheels right from the factory. BMW's X5 V-8 offers an optional sport package that includes 18-in. wheels in front and 19s in back. These are all low-profile tires designed for performance pavement use, regardless of what it says on the sidewall.

A Plus conversion uses a larger wheel (plus X number of inches over stock diameter) and lower-profile tire to give better street performance from the same height tire: for example, the 255/65HR16 on a recent Range Rover 4.0SE becomes a 255/55HR18 like that used on the 4.6HSE. This example uses the same width tire, but you can also increase width (as clearance allows) so you might go from 255/65HR16 to 285/50HR18. Toyo goes out to 23-in. tires for its performance Proxes line and recently introduced a 33x13.0R18 all-terrain for those who want a flotation tire on a large-diameter wheel.

A lower-profile tire sharpens steering and braking, is often stickier and has more grip, and usually detracts slightly from ride quality and adds to road noise. Low-profile tires must be used with caution off-road for three reasons: one, lowering air pressure if you get stuck doesn't enlarge the contact patch as well as it does on standard tires; two, the wheel is closer to the ground so it's easier for small pebbles, sand, or other debris to get lodged in the bead, which will eventually cause an air leak, damage the bead, or both; and three, there's less sidewall and flexibility to wrap the tire around rocks, roots, or downed branches for traction.

If you decide to change tire size in any fashion, be cognizant of downstream considerations. Larger tires, even if they don't run afoul of bodywork, may stick out to the point they're illegal. Bigger generally means heavier, with more stress on wheel bearings, ball joints, steering pumps, and so forth, and heavier rotating mass means longer stops. Finally, taller tires will change gearing and speedo/odo readings, and if the change is more than about 10 percent, driveline inputs to stability and anti-lock systems will be affected and those systems function differently or not at all.

When changing tires and/or wheels, get five (or seven for dualies) if at all possible. This should be a requirement for 4WD, because, by definition, 4WD wants to drive all wheels the same, so having different sizes will defeat the purpose. Remember that regular maintenance (pressure, alignment, lug-nut torque, cleaning) will affect tire life as much, or more than, your driving style.

Air Pressure
With increases in weight and speed, tire pressure hasn't become more important, it's simply that more people are aware of it now. Just as you should check oil and transmission fluid on a routine basis, tires should also be checked. Make a habit of checking them at least once a month. A decent pressure gauge is relatively cheap.

Tire pressures given for a vehicle are always specified for cold pressure and will be a few psi higher when checked hot (from 4-10 psi on average). The pressure listed on the sidewall load-rating is the maximum for that tire and not what it should be run at on your vehicle. A standard size tire should be run at the pressure specified by the vehicle manufacturer, and it should be adjusted for load and conditions as outlined in the owner's manual.

On SUVs, axle loading doesn't change too much under normal conditions. However, if you add a load of cement to the cargo area or attach a trailer, some air adjustment may be called for. And if you check your geometry or physics book, you'll find that 500 lb added to the tow ball increases more than that to the rear axle (and tires) because of the lever effect. Check the trailer tires, too.

For pickups, rear-tire pressure may vary considerably with load; the front will vary less, unless you add a large camper. A pickup with Load Range D tires (max load 2900 lb at 65 psi) may need only half that pressure when empty, and the tires will last longer and ride better. Before and after loading your pickup and trailer, a trip to the local scales will give front axle, rear axle, and total weight for a nominal fee.

If you change tires on your truck to something larger or of different style, start with air pressure recommended by the dealer or manufacturer. To fine tune it to your application, try this method: Drive until the tires are up to normal operating temperature, park on a smooth, flat area, and draw a line across the tread (parallel to the axle) at a few points on each tire with a paint brush or piece of chalk. Slowly roll the vehicle straight forward a few lengths, and check the marks on the tires. Paint worn off at the outside edges means the pressure is too low for the load; paint gone in the center means the pressure is too high for the load. If the marks wear off evenly across the tread, pressure is close to ideal. Verify this over time with tread-depth measurement.

Another approach is to calculate weight on the tire (from your scale axle weights) as a percentage of its maximum capacity, then set tire pressure to that same percentage of maximum air pressure.

With either method, once the pressure is correct front and rear, let the tires cool to get repeatable numbers, and use the same pressure gauge for routine checking as gauges often vary.

Pressure changes relate not only to load, but also according to conditions. Some manufacturers recommend adding 2-5 psi for long, highway trips, and up to 10 psi more for LT tires operating above 65 mph. At the other extreme, air down for sand and less so for other off-highway travel. Improvements in ride softness, traction, and rim protection more than outweigh the minimal loss of ground clearance at lower pressures. Most manufacturers recommend a minimum of 16-20 psi, but experienced off-roaders run substantially less, with big-tire Jeeps and Land Cruisers often down to 2-3 psi. Whenever you lower pressure, always return it to standard for pavement and speed.

Measurement Reality and Relative Gearing
After reading a sidewall, you might expect to have all the numbers you need. But every manufacturer's 31x10.50 or 245/75-16 are not the same size. Though section width of the tire (not including lettering or curb guards) is on the sidewall, tread width is not. The tread will be narrower than section width, though the difference usually decreases with profile.

Accurate tread width is often given in the tire-manufacturer's catalogs, along with data for true diameter, weight, section width, etc. The data listed are based on a specific rim width, though all acceptable rim widths should be listed.

Perhaps the most important numbers in the specifications are rpm (revolutions per mile) and static loaded radius. The rpm figure will allow you to make gearing calculations for 60 mph, when one minute equals one mile. This simplifies speedometer testing, changing to larger tires and axle ratios, and gives more accurate results than using mathematics and dimensions: C=D (C=circumference, D=tire diameter in feet), 5280/C=rpm, but this method doesn't consider that a tire is not a solid object-- it's an air cushion that changes radius as it revolves.

Static-loaded radius is the height of the tire from ground to center, at pressure, with a rated load on it. This figure takes into account the deflection of the sidewall that you see as a bulge, providing a better indication of potential ground clearance and a more precise number for mathematical computation. If you have no data or rpm figures, measuring your tire from the center of the hub to ground with the truck parked on it will enable a good estimate of tire rpm. From there, you can get an idea what will happen to your effective gear ratio when you swap to taller or shorter tires.

Which Wheels and Why
Any change in tire size may require a change in wheel, and a "Plus" conversion will demand it. The wheels you choose may be aluminum (or similar alloy) or steel, with other materials available but rarely DOT/street legal. Whatever wheel you choose, the following must be considered.

Size: Does it have the right offset and backspacing for your truck? If it doesn't, it may make the tire contact the bodywork at extremes of steering lock and/or suspension travel and may wear out wheel bearings and bushings much faster.

Clearance: Does the wheel clear the brake caliper or drum, including interference caused by wheel weights? More than one truck has gone unbalanced as it left the shop with the brake caliper scraping off the balance weights. Tire clearance should be checked at full steering lock and full range of suspension travel. Check in back, too, as a crossed-up solid axle will sometimes make an oversize tire rub on the inside wheelwell.

Lug nuts (or bolts): Aluminum wheels are normally thicker than steel wheels and therefore may need a longer wheel stud or bolt. The type of lug nut (conical, shank, flat, etc.) may also vary by wheel. If you purchase four new wheels (because you were cheap or the new tire won't fit in the spare well), be sure to keep whatever bolts/lugs you need for the spare in case of a flat. Lug-nut torque is often different for aluminum or steel wheels, dual wheels sometimes have higher torque specifications, and aluminum wheels in particular should be retorqued a hundred or so miles after installation.

Valve stems: Expensive cars come with metal valve stems, and many people add them with new-wheel purchases. Metal stems rarely rot like rubber, but they do not bend either, so if your wheels often encounter downed tree branches, offensive rocks, or chunks of ice and snow, rubber stems will yield and may be better in the long run.

Multipiece: Two- and three-piece wheels are popular for high-performance cars, but they aren't recommended for applications with tires over 31 in. or 265/75 because of the high rim loadings and weight of the average new SUV or light truck.

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