What's the difference if cars have bigger tires?

What's the difference if cars have bigger tires? For example, I notice sports cars usually have 17- or 18-inch tires. "Ordinary" cars usually have 15-inch tires. Upper trim levels of these cars get bigger tires. I heard bigger tires enable a car to go a further distance in a same amount of time, because of the bigger circumferences. Bigger tires can stop a car better because of wider surface contact. But bigger tires need more gas, possibly more road noise. What are all the pros and cons of bigger versus "normal"-size tires?

Kenneth, Staten Island, N.Y.

Tire and wheel size play a large part in the overall feel and handling of your car. Here’s an explanation from Cars.com senior editor Joe Wiesenfelder on tire and wheel sizes and how they affect your car:

“Well, there are several questions in your question about this surprisingly complex aspect of motoring.

“To begin with, changes in tire size — that’s the full diameter of the tire itself, measured from the tread — are rare. Going with a larger or smaller tire diameter throws off the speedometer and the antilock brakes and stability system calibrations, if your car has them. It’s unwise and unsafe. Increasing tire diameter also can be unsafe because it makes your brakes work harder to stop the car ... I could go on. A few automakers have taken a piece of the upgrade pie by offering larger-diameter tire-and-wheel packages that have been run through all their safety and durability testing. A certified dealer installs them and recalibrates the speedometer, ABS, etc. at the same time. This is the only guaranteed (and warranteed) safe way to do it – if the parts and program are truly from the manufacturer and not the dealership itself. (Dealer modifications may not be covered by the original warranty.)

“There are more options when you leave the tire diameter the same. In some cars you can switch to a slightly wider tire without changing anything else — say, from P195/75R14 to P205/60R14. (See here for a dissection of tire codes.) Note that the first number, called the section width, is the only actual size that changes. The sidewall is the same height, but it’s expressed as a percentage of the tire’s width, so it’s now smaller (60) in comparison. Because the other dimensions don’t change, the amount of tread touching the ground widens and, if it’s the same tire model, improves traction on dry and possibly wet surfaces. It would diminish snow performance, though, as wider tires do (counterintuitive but true).

“What most automakers offer when you buy a car qualifies as plus-sizing. Plus-1 means the wheel’s diameter is 1 inch larger than the original, and the tire’s sidewalls are shorter to make up for it, keeping the overall tire diameter the same. Usually the tire’s width also increases somewhat when you do this. Plus-2 is the same idea but a 2-inch wheel increase. Once you get above plus-2, the less likely a change will work on a car. Just about any dealer or tire store can help you plus-size the wheels and tires on any car.

“There are only a few pros and cons that are absolute. Plus-sizing means the tire sidewalls are shorter, so there’s less rubber deflection. In other words, they don’t absorb bumps as well, and the car changes direction more directly when you steer. This sharper “turn-in” also results from a change in the contact patch, which is where the tread touches the ground. As sidewalls become shorter, the contact patch becomes wide but not as long from front to rear, and that gives the steering a more immediate and connected characteristic. Improved roadholding is not a guaranteed result. The downsides include a rougher ride, faster suspension wear (something needs to absorb those bumps!) and more potential for rim damage.

“All other things being equal, an upsized tire would hurt gas mileage, but other things are seldom equal. A narrow tire can be louder than a wide one ... and have better traction ... and last longer ... and provide worse gas mileage. It’s all about the tire. Here are some of a tire’s characteristics: dry traction, wet traction, ice traction, snow traction, hot traction, cold traction, road noise, rolling resistance, treadwear, load rating, temperature rating, speed rating, ride comfort and run-flat/non. Those are the ones I can think of. No tire does all these things well, but the models that do the most the best ... well, they tend to cost you.”

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Answered by Joe Bruzek on September 7, 2007 in How Does That Work? | Permalink

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