Many models have optional tire sizes that are usually part of a package. There is a sticker typically found at the rear of the driver's door opening that will list the original tire size.
As a suspension and alignment specialist, the issue that is most important to me is a non-adjustable alignment angle called "scrub radius". When you look back to the front of the car, and draw an imaginary line between the two steering pivots for one wheel, that line will bisect the tire tread where it hits the road at a very specific point. For cars with front struts, the upper steering pivot is the upper strut mount. The lower pivot is the lower ball joint.
Scrub radius makes the left part of the tire tread want to tug the steering system to the left, and the right part of the tread tug to the right. The two forces offset each other, but they are modified slightly to design in handling and braking characteristics. Scrub radius changes when the tire is larger in circumference than it should be, if the wheel is offset more than it was originally, or in some cases the wheel is wider than the original wheel.
17" and 18" tires can have the same outer circumference and offset, but the sidewalls will need to be different. The people at any tire store should be able to tell you which optional sizes can be used on your car. If they look confused when you bring up "scrub radius", consider finding a different store.
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Monday, April 17th, 2017 AT 4:40 PM
