Problems With a Tire Size Different From the Factory Size
- Taller tires offer increased ground clearance and potentially a bit more traction, but you may run into a few problems with them. As off-roaders know, the biggest problem with using taller tires is getting them to fit into the stock wheel well without hitting the fender lip or the wheel tub when the suspension compresses. Taller tires will also increase your car's aerodynamic front -- a detriment to top speed and fuel economy -- and raise its center of gravity. A higher center of gravity means more lean in the corners, more strain on the shocks and springs and slower handling response.
- Traction increase gained from using wider tires is a double-edged sword, since you have to think about not only how much traction you get but where it's going relative to the steering axis. Changing wheel offset -- the distance from the wheel mounting flange to the centerline of the rim -- can move the wheels farther outward for increased suspension clearance, but it'll also move the wheel centerline relative to the ball joints. The wheel centerline exerts a levering pressure on the steering axis; move the wheel centerline closer to the ball joints, and you reduce steering effort at the cost of steering feedback and precision.
- "Diameter staggering" is the practice of using taller tires either on the front or rear axle and shorter tires on the other. Using taller tires on only one axle, without lowering the suspension to compensate, will lift that end of the car, causing it to roll more while handling than the end with smaller tires. The tall-tire side will take on different handling characteristics from the short-tire side, possibly resulting in a pendulum effect during back-and-forth transitions if the tires are on the rear. Staggering diameter will also confuse your anti-lock braking computer, tricking it into thinking that the slower-spinning big tires are on the verge of lockup. The computer will then route braking force away from the larger tires and toward the smaller ones.
- Width staggering is all about balance. Using wider tires won't upset most computers because they don't know or care how wide your tires are -- they're either the same diameter or they're not. Some stability and AWD control systems might have a slight issue with them because they alter front-to-rear traction characteristics, but it shouldn't be beyond the system's ability to compensate. Using wider tires on the front of a front-drive car is rarely a bad thing, because the front wheels do 80 percent of the work, but wider tires on the back of a front-drive are nonsensical. Wider tires will work on either end of a rear- or all-wheel-drive car, placement depending on whether you want to increase traction for steering response, handling and braking, or acceleration and stability.
- Aspect ratio is the size of a tire's sidewall relative to the width of the tread, with the sidewall height expressed as a percentage of the tread width. If you've got 10-inch-wide tires and 5-inch sidewalls, you've got an aspect ratio of 50, or 50 percent. All else being equal, increasing wheel and tire width alone will decrease aspect ratio for quicker handling response and better traction while turning, while increasing tire diameter alone will increase aspect ratio for a softer ride and better straight-line traction. Recall the pendulum effect of diameter staggering; the tall tires' increased aspect ratio and subsequent loss of turning response contribute heavily to this phenomenon.
