by 
The Curious Bag
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Knowing how to decode the alphabet soup tire ratings can make a crucial difference in your car's performance and safety. Those numbers and letters carry vital information about the tire's grip, capacity and estimated service life.
Size Rating System
Tire size is described in the sequence on the side of your tire that begins with a letter. An example would be P225/65/R15.
Width
The three-digit number that comes first in that series is the tire's width in millimeters.
Aspect Ratio
The two-digit number that comes next in the series tells you the height of the tire's side-wall height divided by its width. A low-profile performance tire might be rated 20, and a heavy-duty truck tire might have a 70-Series rating.
Rim Size
The last in the series begins with an "R" and tells you the size of rim the tire is designed to fit.
Load Index
The two-digit number clockwise from the tire size is the load index, which tells you how much weight the tire can carry.
Speed Rating
The letter attached to the load rating is the top speed rating, and ranges from M to Z. In general, letters further down in the alphabet mean a higher top speed rating.
Source:
Swedishbricks.com: Tire Questions
Tiresplus.com: Tire Safety FAQ
Resource:
Video: How to Interpret Tire Ratings
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