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Some of this has already been said but there were also a few issues so I am going to start at the beginning.
Example tire size:
195/60R 15 88V
In the case of our example tire:
* As pointed out 195 is the width of the tire in mm.
* 60 is not the height of the tire but as GM correctly points out the height of the tire as a percentage of the tire's width. So a 195/60 tire would have a sidewall height of approximately 117mm.
* 15 is the diameter of wheel in inches that the tire fits.
* R simply means radial construction. Almost all modern tires are radials.
* 88 is the load index. The load index indicates the max load carrying capacity of the tire when properly inflated. You can look up the corresponding weight for the index value in tire tables. It is important to make sure the load index is appropriate for your vehicle.
* V is the speed rating of the tire. The speed rating is simply the max speed the tire can sustain for a certain length of time when properly loaded and inflated without failing under laboratory test conditions. It doesn't indicate anything about tire grip as people often claim although higher speed rated tires generally have better grip.
* The plus size concept is just a convenient way to talk about different size tires that are approximately the same diameter as the original. This way you can get a quick sense on which alternative sizes may work on your car without disrupting your speedometer reading, center of gravity, ground clearance and gearing. However, you still need to be concerned about fitment problems due to tire width and other issues. So for instance, the tire diameter of our example tire is approximately 24.3in. A plus setup maintains the diameter reasonably close. A 205/55 15 has an approximate diameter of 23.9in which is reasonably close. This is referred to as a Plus Zero fitment because the wheel diameter remains the same. A 205/50 16 has an approximate diameter of 24.2in which is very close to our original. However, this fitment requires a wheel that is 1 inch larger in diameter and as a result is referred to as a Plus One fitment.
* The common notion that a wider tire will ALWAYS provide better grip (on dry pavement at least) simply isn't true. The "best" width for such conditions depends on a combination of the wheel width and the suspension setup.
*A shorter sidewall (all other things equal) generally provides quicker steering response and greater lateral grip although at the cost of ride comfort.
Above I repeatedly referred to approximate size. This is because although you can calculate these figures different brands and models of tires actually vary a fair amount. It is worth consulting the tire manufacture's specifications when changing fitments to find an appropriate tire.
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