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" The most important pieces of real estate on your car are those four small contact patches that grip the road--yeah, your tires. Those few square inches of rubber determine, well, just about everything. Tires harness the engine&amp;#039;s power, allow the brakes to do their job and determine how successfully a car will go around a corner--whether it&amp;#039;s pulling into a parking stall or screaming into a high-speed sweeper. Yes, tires carry an enormous burden. That thin strip of round and black is a complex assembly of cables and rubbers, molded into a highly engineered profile. It&amp;#039;s then mounted onto a wheel and, of course, inflated with air to give it shape and definition. The manufacturers behind your vehicle, tires and wheels worked together to carefully engineer an optimum combination of grip, road feel, ride quality, noise control and tire wear. Since we&amp;#039;re Americans, a lot of us monkey around with all that hard work and buy new wheels and tires. Used to be that oversize tires were the earmark of a gearhead. Now you see housewives with 20s on their SUVs in every Wal-Mart parking lot. According to Matt Edmonds, TireRack&amp;#039;s vice president of marketing, the plus-sizing market has doubled in the past 10 years as wheel and tire upgrades have migrated to the mainstream from enthusiasts. The goal is to increase performance, right? Well, aftermarket tires and wheels can certainly help if you choose wisely. And our experience has proved that modesty is the best policy when it comes to wheels and tires bigger is not always better. There&amp;#039;s a sizing sweet spot that provides better grip and those head-turning looks, too, without compromising your vehicle&amp;#039;s original engineering. So let&amp;#039;s get some basics down before you upgrade to new rolling stock. Of Wheels and Men The first wheels were probably made of wood, despite Fred Flintstone&amp;#039;s granite-shod convertible. And automobiles used wooden carriage wheels for quite a while. Increased power and weight soon outstripped wood&amp;#039;s capabilities, and wheels were upgraded to steel, either a hub-spoke-and-rim design or a stamped, welded dish. Lightweight steel-spoked wheels lingered until the &amp;#039;50s, especially on fussy little foreign sports cars. But American cars needed the stronger, heavier stamped and welded steel wheels. Steel&amp;#039;s weight penalty led racers and enthusiasts to explore magnesium--a metal as strong as aluminum, yet even lighter. Unfortunately, magnesium corrodes within hours unless it&amp;#039;s painted or regularly polished. Worse, magnesium could catch fire in an accident. And taking a water hose to a burning magnesium wheel doesn&amp;#039;t put out the fire--it simply makes it burn more intensely. Mag wheels were used largely on race cars because of these on-road liabilities. So, what some old-schoolers call &amp;quot;mag&amp;quot; wheels today are actually an aluminum alloy. Modern alloy wheels are almost always painted. Then there are some people out in California who like to chrome-plate that aluminum, thereby increasing sales of sunglasses in their neighborhood. You know who you are. Beyond aluminum, there are ultralightweight carbon-fiber wheels. In fact, wheel manufacturer Dymag sells carbon-fiber rims with magnesium centers. But trust me, you don&amp;#039;t want to know what those cost. So, most wheel upgrades involve aluminum. How Big? There are two ways to increase the size of the tire&amp;#039;s contact patch--make it longer or wider. A longer patch would require the tire&amp;#039;s overall diameter to increase--which makes some sense on a 4wd truck. But increase the diameter of the tire on most passenger cars and you have problems. First, the tire hits stuff (like the fender). Second, because the radius of the overall tire is larger, the effective gearing gets taller, slowing down the engine at any given speed--and robbing your car of acceleration. Third, because of increased angular momentum--weight concentrated near the rim--the ABS calibration goes wacky. (You may not notice this one until you hammer the brakes coming up to some slippery corner and go sliding off into the hedge.) So, on street cars, we customarily go wider. The industry standard is to add an inch, say from a 7-in.-wide rim to an 8-in. That allows a wider tire to be mounted. That wider rim is usually an inch taller, say 17 in. rather than the original 16 in. Then we mount up a tire with a wider tread and a lower profile for better handling. Because of the lower profile, the overall diameter is pretty close to the original. That&amp;#039;s called a Plus 1 upgrade because the wheel is 1 in. taller. Similarly, we can go even lower in profile with a Plus 2 (16- to 18-in. wheels) or Plus 3 (16- to 19-in.) upgrade on most vehicles without running into a problem. Usually. Wretched Excess As the aspect ratio of a tire drops (lower profile), a number of things change. The shorter sidewalls stabilize the tread, improving grip and enhancing road feel through the steering because they&amp;#039;re stiffer and less compliant. That&amp;#039;s good. But it&amp;#039;s not all good: The contact patch becomes more square than oval. The increased width of the tire on the pavement makes the tire more prone to hydroplaning on wet roads. Even at modest speeds, it&amp;#039;s possible for the rubber to ride on top of the water instead of plowing through the water to the pavement. This reduces grip to nearly zero, which is a Very Bad Thing. At the same time, ride quality suffers. One major downside to shorter sidewalls is an increase in wheel damage--those short sidewalls put the rim a lot closer to the potholes and curbs. The short, wide patch has more contact area on the road, but that&amp;#039;s only if the wheel remains perpendicular (or nearly so) to the ground. The suspension&amp;#039;s job has just gotten tougher. A taller, more compliant sidewall has an easier time keeping the contact patch on the ground. With a wider patch and more grip, the car rolls more, lifting the inner part of the tread off the pavement and suddenly reducing grip. So, without retuning the suspension, handling can actually suffer. Many cars use a centering hub, a raised center section of the hub that mates with a matching recess in the wheel. It&amp;#039;s intended to keep the wheel precisely centered on the hub, more precisely than by just tightening the lug bolts. Some wheels may not fit this hub properly, requiring the use of a spacer or even a different wheel. The new wheel has to have the correct offset to clear the suspension and brakes. If the rim is wider than stock, there may not be enough clearance to the ball joint or steering arm to permit half the extra width to go inside the wheel well. And adding inches to the outside upsets steering geometry and overstrains wheel bearings. And, oh yeah, it makes the tire sidewall rub the fender. Cars make plenty of good noises, like a throaty exhaust. But tire rub is not one of them. So take an integrated approach to upgrades--and measure everything twice. We see mistakes on the road all the time. Don&amp;#039;t get me started on that weird stuff on MTV. A giant SUV with chromed 22-in. wheels and 25-series tires might look fly, but I guarantee that truck doesn&amp;#039;t work very well. The super-low-profile tires make it ride like, well, a truck. Or maybe like Fred&amp;#039;s aforementioned convertible, which did have ultra-low-profile rollers. The rims won&amp;#039;t survive most pothole-infested urban streets and the vehicle is nearly undriveable in the rain. Imagine replacing the stock wheels on a Honda Civic with much larger ones. You&amp;#039;d spend a fair amount of money and effort retuning the chassis to take advantage of the extra rubber; the additional unsprung weight would simply overwork the factory springs and shocks. In addition, the extra mass concentrated in the rim and tire would require a disproportionate amount of power to accelerate. So, in essence, the Civic would be slower off the line--and slower in the corners. Quite an &amp;quot;upgrade,&amp;quot; eh? Our advice: Purchase a proven wheel-and-tire combo specifically designed for your vehicle. Check places like tirerack.com for a myriad of options, or go to a shop that specializes in your kind of ride. Parsing the Sidewall What those numbers on the side of your tire mean.     1. Prefix P = Passenger car T = Temporary spare LT = Light truck ST = Trailer   2. Section width That`s the overall width of the tire in millimeters when mounted on the correct width rim.     3. Aspect ratio This indicates the distance from the wheel-rim mounting surface to the outside of the tread. Aspect ratio is expressed as a proportion of the width instead of as an absolute number, so it&amp;#039;s easier to compare the profiles of different-size tires. A 50-series tire is half as tall as wide, while a 70-series is 70 percent as tall as its width.   4. Speed rating Only the highest-rated tires, the Z-rated models, still label the speed rating inside the tire-size designation. On all other tires, it&amp;#039;s found elsewhere on the sidewall.     5. Construction R = Radial-ply tire.  D = Refers to diagonal, or old-fashioned bias-ply tire, now used only on trucks and trailers.    6. Size The diameter of the rim in inches. If it seems odd to you to spec tire width in millimeters and diameter in inches, it does to me, too. A few special-purpose tires, like PAX system tires, use a metric designation for rim diameter.    Don&amp;#039;t Get Rubbed the Wrong Way Maintaining Aedquate Clearance  Plus Sizing A 1-in.-larger rim diameter and 1-in.-wider rim width is a &amp;quot;Plus 1&amp;quot; size upgrade; in this case, the 16-in. rim below. Plus 2 and 3 are shown, slightly exaggerated, on the 17- and 18-in. wheels. Choose a tire that&amp;#039;s close to stock height.  "^^<http://www.w3.org/2001/XMLSchema#string>
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