Can You Change the Size of a Car Tire?

Written by richard rowe
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Can You Change the Size of a Car Tire?
Plus-sized tire/wheel packages like this enhance handling and high speed stability. (sports car rim image by Lario Tus from Fotolia.com)

Generally speaking, most people seeking to change tire size will replace their tire with taller and/or wider rubber for increased performance or a new look. Taller tires increase ground clearance, which is a make or break thing for off-road trucks and can have some surprisingly beneficial affects on road-going racers as well. Most performance applications will use wider tires to put more rubber on the road, but thinner tires have their place, too.

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Electronic Probems

Before considering a tire swap, consult your manufacturer to see what your chassis can tolerate. Taller tires can wreak havoc on a car's anti-lock braking systems, traction control systems and stability control. This is especially true when you use larger tires on one axle than the other, as is common for older muscle cars. Larger diameter tires have a higher circumference than small units, so they move further per revolution. The computer reads this as wheel-slip, and will change braking, traction and stability settings to counteract it.

Increasing Width

Tire grip rises or falls almost linearly with tire width. If one tire is twice as wide as another, it should have twice as much mechanical and molecular grip on the road surface. In reality, the rise isn't quite linear, especially in the wet; wide tires tend to trap water underneath, necessitating wider water channels. These wide water channels make for less rubber on the road, mitigating some of the fat tyre's gain. Wide tires, however, spread the car's load out over a wider area, allowing you to use a tire with a softer and "stickier" rubber compound without significantly reducing durability.

Diameter

You can use as tall a tire as you can physically fit in the wheelwell, provided you keep a few things in mind. First, remember that most cars have at least 2 to 3 inches of suspension travel, so make sure the tire is at least that far from anything it can hit. Aside from the electronic issues mentioned before, you'll also need to recalibrate your speedometer, since it will read slow when using larger tires and fast when using smaller. You should never use different diameter tires front and rear on 4WD or AWD cars. These cars are engineered with the understanding that both axles will always turn at the same speed. Using different sized tires will almost certainly result in a broken transaxle or centre differential.

Plus-Sizing

Large diameter wheels have become an extremely popular modification for vehicles of all types. This styling and performance trend has led to what is called "plus-sizing," which means using larger diameter wheels inside of tires the same diameter as the originals. Of course, this means replacing all four wheels and tires, but keeps the electronic nannies at bay. Plus-sizing results in a vehicle with very short tire sidewalls, which improves handling and steering response at the expense of ride.

Diameter for Road Use

Off-roaders have used taller tires for increased ground clearance for more than a century, but (when combined with lowering and plus-sizing) slightly larger diameter tires can benefit street cars as well. Larger diameter tires move the tyre's centre further from the ground, which can help make the body less likely to roll while cornering. The trick is to lower the car in accordance with the rise in tire diameter (lower the car 1 inch for every 2-inch increase in tire diameter), and to plus-size the combination to keep the larger tire from flopping around under load.

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