Purcell: We'll keep you moving

Tire Information/Terminology

The tire information provided will give you a better understanding of all aspects of your tire.

• Aspect Ratio
• Heat Cycle
• How a Tire is Made
• Load Index
• Plus One Concept
• Speed Rating

Tire Aspect Ratio -The ratio of section height to section width. A 70 series tire’s height is equal to 70% of its width. Lower profile tires have a lower aspect ratio. In general, tires with a lower section height (i.e. where the aspect ratio is lowered) have a higher cornering force and therefore improved performance and handling characteristics.

Load Index - Many tires come with a service description added on to the end of the tire's size. These service descriptions contain a two-digit number which includes the load index and speed rating letter. The load index is a representation of the maximum load each tire is designed to support.

Plus One Concept - The plus one concept is the proper sizing up of a wheel and tire combination. Each time you add 1 inch to the wheel diameter, add 20mm to the tire width and subtract 10% from the aspect ratio. This compensates for the increases in rim width that generally accompanies the increases in diameter too. By using a larger diameter wheel with a lower profile tire, it is possible to properly maintain the overall rolling radius, keeping the odometer and speedometer changes negligible. However, be prepared for a stiffer ride as there will be less sidewall to cushion undulations in the road.

Speed Rating - All tires are rated with a letter representing speed capabilities. This indicates the maximum speed that the tire can sustain for a ten minutes without coming to pieces and destroying itself.

Use the following chart to determine the maximum load-carrying capacity based on a tire's load index:

Letter   MPH KM/H

P 95        150

Q 100      160

R 106      170

S 112      180

T 118      190

U 124      200

H 130      210

V 150      240

W 168      270

Y 188      300

Z 149+    240+