Understanding Tire Construction
Aspect Ratio

Cross Reference Chart
This chart does not imply complete interchangeability.
Pre -1964 '65 to '7280 series
metric		Alpaha
Numeric
78 Series		P - Metric 75
series Radial		P-Metric 70
series Radial
590-13
640-13
725-13		 600-13
650-13
700-13		 165-13
175-13
185-13		 A78-13
B78-13
D78-13		 P175/70R-13
P185/70R-13
P205/70R-13		 P165/75R-13
P175/75R-13
P185/75R-13
590-14
650-14
700-14
750-14
800-14
850-14		 645-14
695-14
735-14
775-14
825-14
855-14		 155-14
175-14
185-14
195-14
205-13
215-14		 B78-14
C78-14
E78-14
F78-14
G78-14
H78-14		 P175/75R-14
P185/75R-14
P195/75R-14
P205/75R-14
P215/75R-14
P225/75R-14		 P185/70R-14
P195/70R-14
P205/70R-14
P215/70R-14
P225/70R-14
P235/70R-14
590-15
640-15
650-15
670-15
710-15
760-15
800-15
820-15		600-15
685-15
735-15
775-15
825-15
855-15
885-15
900-15		165-15
175-15
185-15
195-15
205-15
215-15
230-15
235-15		A78-15
C78-15
E78-15
F78-15
G78-15
H78-15
J78-15
L78-15		P165/75R-15
P175/75R-15
P195/75R-15
P205/75R-15
P215/75R-15
P225/75R-15
P235/75R-15
P235/75R-15		P175/70R-15
P185/70R-15
P205/70R-15
P215/70R-15
P225/70R-15
P235/70R-15
P235/70R-15
P255/70R-15
To figure inches divide 25.4 into the metric size. To figure metric multiply the inches by 25.4
Aspect Ratio/Series Number - the height and width relationship of the tire. This number is determined by dividing a tire's section height by its section width. For example: the section height of G70-14 is 70% of the section width

Section Height - the the tire's section measured from the rim seat to the outer thread surface of an inflated tire.

Cross Section - the linear distance between the exteriors of the sidewalls of an inflated tire.

Bead - the specialized area of the tire designed to mate with the rim flange. Reinforced with steel cable on straight side tires.

Bead Seat - the horizontal surface of the rim where the bottom of the tire contacts the rim.

Rim Diameter - the diameter of the rim at the bead seat. The most accurate method to determine the diameter is to measure the circumference of the rim at the bead seat and divide by (3.14)

Tread Width - the portion of the tread design that comes in contact with the road surface.

Overall Diameter - the linear distance between the exterior tread surface from the ground to the top of the tire.

Load Capacity - the maximum weight a tire can support at its maximum inflation pressure (i.e. 1360 lbs @ 32 psi).

Speed Ratings
P =93 MPH  
S =112 MPH 
T =118 MPH 
H =139 MPH 
V =149 MPH 
Z =+149 MPH 

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