Wheel and Tire Guide - Get a Grip, Wheel Ya?

Speed Ratings

A tire earns its speed rating based on lab testing, during which it is pressed against a drum (to approximate a load) and then run at increasing speeds until its required speed has been met. That required speed is expressed as a letter (see below).A few years ago it was believed that a Z rating (149-mph plus) was the highest a tire could achieve. But with the recent introduction of supercars that can surpass that number, new W and Y ratings were developed.

Rating Speed

Load Index

This is a number assigned to a tire that represents its ability to carry a certain amount of weight. The number has no relation to the actual load capability, it's merely a number that's relative to others in the index. There are too many load index numbers to list them all here, so if you're interested you should be able to find this data on any tire maker's site.

Note that a particular brand, type, and size of tire can have different load ratings. For example, a 205/55R16 Goodyear Eagle RS-A high-performance tire comes with either an 89 rating (1,279 pounds) or a 91 rating (1,356 pounds). When choosing a replacement tire, make sure you either equal or go above the OE load rating index, or you risk overheating your tires by making them work too hard to carry the weight of your vehicle.

By Drew Hardin

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