ATLcrusher said:
Am I confused? I thought 285 or 305 was a measure (in mm) of the width of the tread, not height? And 70 or 75 (or whatever) was a measure of the sidewall as a percentage of the tread - i.e. a 285/75 would have a sidewall of approximately 213mm (75% of 285).
Actually the 285/295/305 etc is measured from sidewall to sidewall, not tread width, not sure on american imperial specs but I think if a tyre is 12.5, that is the sidewall specs not the tread width
Passenger Tyre Sidewall Information
Example: P215/65R15 89H
"P" means this is a passenger car tyre (as opposed to a tyre made for a truck or other vehicle). P-metric is the U.S. version of a metric tyre-sizing system.
"215" Section Width: The width of the tyre in millimeters from sidewall to sidewall. This measurement varies depending on the width of the rim to which the tyre is fitted: larger on a wider rim, smaller on a narrow rim. The number on the side of tyre indicates the width measured with the tyre fitted to the recommended rim width.
"65" Aspect Ratio: The ratio of height to width; this tyre's height is 65% of its width.
“R" Construction: How the plies are constructed in the tyre carcass. "R" means radial. "B" in place of the "R" means the tyre is belted bias construction. "D" in place of the "R" means diagonal bias construction.
15" Rim Diameter: The diameter of the wheel in inches.
"89" Load Index: This tyre has an industry-standard maximum load of 580 kilogram. Different numbers correspond to different maximum loads. The maximum load is shown in lbs. (pounds) and in kg (kilograms), and maximum pressure in PSI (pounds per square inch) and in kPa (kilopascals). Kilograms and kilopascals are metric units of measurement.
"H"Speed Rating: This tyre has an industry-standard maximum service speed of 210 km per hour. Tyres using an older European system carry the speed rating in the size description: 215/65HR15. Different letters correspond to different maximum service speeds.
The letters "DOT" certify compliance with all applicable safety standards established by the U.S. Department of Transportation (DOT). Adjacent to this is a tyre identification or serial number; a combination of numbers and letters with up to 11 digits.
The sidewall also shows the type of cord and number of plies in the sidewall and under the tread.
Tyre Grades: Uniform Tyre Quality Grading System or UTQG
Except for snow tyres, the DOT requires tyre manufacturers to grade passenger car tyres based on three performance factors: tread wear, traction and temperature resistance.
Tread Wear
# More Than 100 - Better
# 100 - Baseline
# Less Than 100 - Poorer The tread wear grade is a comparative rating based on the wear rate of the tyre when tested under controlled conditions on a specified government test track. A tyre graded 200 would wear twice as long on the government test track as one graded 100. Your actual tyre mileage depends upon the conditions of their use and may vary due to driving habits, service practices, differences in road characteristics and climate. Note: Tread wear grades are valid only for comparisons within a manufacturer's product line. They are not valid for comparisons between manufacturers.
Traction
# A - Best
# B - Intermediate
# C - Acceptable
Traction grades represent the tyre 's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. The Traction grade is based upon "straight ahead" braking tests; it does not indicate cornering ability
Temperature
# A - Best.
# B - Intermediate
# C - Acceptable
Load Index and Speed Rating
The temperature grades represent the tyre's resistance to the generation of heat when tested under controlled conditions on a specified indoor laboratory test wheel. Sustained high temperatures can cause the materials of the tyre to degenerate and thus reduce tyre life. Excessive temperatures can lead to tyre failure. Federal law requires that all tyres meet at least the minimal requirements of Grade C.
The speed rating is the maximum service speed of a passenger car tyre. Light truck tyres are not speed rated. Here is a list of rating indicators and their mile-per-hour equivalents. This rating system applies to all tyre makers.
Rating Maximum Speed
Q 99 mph
S 112 mph
T 118 mph
U 124 mph
H 130 mph
V Above 130 mph (without service description)
V 149 mph (with service description)
Z Above 149 mph
Light Truck Sidewall Information
"LT" stands for Light Truck
"LT235/85R16" is the size designation for a metric light truck tyre
"LOAD RANGE D" identifies the load and inflation limits
"RADIAL" indicates that the tyre has a radial construction
"MAX. LOAD SINGLE 2623 lbs. AT 65 psi COLD" indicates the maximum load rating of the tyre and corresponding minimum cold inflation pressure when used in a dual configuration. The other markings on the sidewall have the same meaning as described for the passenger car tyre.
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