Passenger Tire Sizing

P-Metric
Examples: P215/65SR15, P215/65R15, P215/65R15 95S
P = Passenger Car Tire
215 = Section Width in Millimeters
65 = Aspect Ratio
R = Radial Construction
15 = Rim Diameter in Inches
95S = Service Description (Load index and Speed Rating)

European Metric
The aspect ratio is assumed to be 82 if not present.
Examples: 155SR13, 155R13, 155R13 78S
155 = Section Width in Millimeters
S = Speed Rating
R = Radial Construction
13 = Rim Diameter in Inches
78S = Service Description (Load index and Speed Rating)

With the aspect ratio included:
Example: 185/70SR14, 185/70R14, 185/70R14 88S
185 = Section Width in Millimeters
70 = Aspect Ratio
S = Speed Rating
R = Radial Construction
14 = Rim Diameter in Inches