Tire designation systems currently in uses are:
P-Metric European Metric LT-Metric Alpha-Numeric
Numeric Floatation T Type Temporary Spare
Letters used in tire designations are:
R - Radial Construction B - Belted Bias Construction
D - Diagonal Construction P - Passenger Car
T - Temorary Spare LT - Light Truck
Examples of tire designations:
P-Metric - P 205/ 70 R - 15
15 = wheel diameter stated in inches
R = indicates radial-ply construction
70 = profile ratio percentage
205 = sidewall to sidewall width
P = indicates passenger car tire
LT-Metric - LT 215/ 85 R - 16
16 = wheel diameter stated in inches
R = indicates radial-ply construction
85 = profile ratio percentage
215 = sidewall to sidewall width
LT = indicates light truck tire
The outside diameter of a P 205/70R-15 tire is determined as follows:
O.D. = width x ratio x 2 + wheel diameter
= 205mm x .70 x 2 + [15in•(25.4mm/in)]
= 668 mm
= 0.668 meters
The actual rolling diameter is about 95% of the true outside diameter. This is what should be used to determine the rolling circumference of the tire (for our sample tire.)
Tire Circumference = O.D. x .95 x _
= 1.99 meters
and Tire Revolutions per Kilometer = (1,000 meter)/(1.99 meters)
= 502.5 revolutions per km
A more accurate method to determine Tire Rolling Radius is to measure the actual distance from the hub center to the road of the completed vehicle.
A coefficient for the rolling resistance for each tire may or may not be available from the manufacturer. In general most tire rolling resistance coefficients fall under these approximations:
1970 era Bias-ply tires 0.020
1980 era Radial tires 0.010
1990 era Radial tires 0.010 to 0.007
Typical 1990 U.S. Car 0.008
Goodyear Tires on 1990 Impact 0.0048
GM Sunraycer bicycle tires 0.0037
The rolling resistance from the tires is found by multiplying the weight on a tire by the rolling resistance coefficient. The following example is for the GM Impact:
FT = vehicle weight x rolling resistance coefficient r0
= 9,790 N x .0048 = 48.95 N (48.95 N = 11.0 lbf)