Tire Ratings and Sizes
The tire sizing system is broken into easy to understand sections that describe sizing, load, and speed ratings. Its important to install the correct tires on a vehicle and to keep the correct air pressure in all four. Different sized tires can effect the ABS braking system as well as effecting the vehicles safety and driveability. 
We will use the random tire size P225/60R16 97H to help describe the meaning of these different sections. The "P" found on most tires stands for the P-metric size. P-metric means that it is fitted for passenger vehicles like cars, light trucks, SUV's, and mini vans. They will usually have a 1/4 ton and 1/2 ton load capacity. Light trucks will have the LT prefix which stands for light truck metric. These tires will typically have a 3/4 ton to 1 ton capacity.
Sometimes there is no letter at all preceding the three digit number. This means that the tire was designed for European vehicles and may have slightly different load capacities. If the series ends with a C the tire was designed for heavier load capacities.
The next three digits "225" following the P-metric are the tires section width. The section width of a tire is the measurement of the mounted tires widest part between the inner sidewall and the outer sidewall. This number is indicated in metric millimeters and can be converted to inches by dividing by 25.4.
Following the section width is a two digit number that indicates the tires profile or sidewall aspect ratio. This section indicates the tires height from the edge of the tires rim to the tread. This number is a percentage of the tires section width. For instance the tire we are using has a section width of 225 millimeters divided by 25.4 = 8.86". To find the tires profile simply multiply the section width by 60% or .60 to get 225 / 25.4 *.60 = 5.31". This tire has an aspect ratio of 5.31 inches. Aspect ratio is sometimes referred to as section height.
The next section will typically be an R because most tires sold today are radial tires. Yes "R" stands for radial. A "D" indicates the tire is a diagonal bias-ply which are not as common as they used to be. This is followed by the size of the tire or rim the tire will fit onto. This number is usually in inches "16" but can be in millimeters "390" on bigger tires.
The last three digits indicate the tires service description. The first two digits are the tires load rating which indicates the tires load carrying capacity. The last letter is the speed rating. Speed ratings are listed above and it should be noted a Z rated tire indicates only that a tire is capable of 149 and over where the more recent W rating is rated at 168 and the Y 186.
