E.T.R.T.O. MARKINGS:
32 x 590 (650 x 32A)
Of the primary methods of determining tire size, this is the most practical and
precise.
The first number (32) designates, in millimeters, the width of the tire.
The second number (590) designates, in millimeters, the bead seat diameter of
the tire.
Unlike automobile or motorcycle tires that use actual rim diameter at the bead
seat area to determine the size (example: Automobile 195/65R
”15” or Motorcycle 180/55ZR”17”) bicycle tires derived their sizes from the outside diameter of the tire. As specific diameter (actual) and section width tires became common, various
size (section width) tires were created to fit these popular rims.
These tires were no longer at the same outside diameter of the original. As some of these new sizes became common the sizing system became really
confusing since the size was not based on the actual outside diameter of these
newer tires.
MARKINGS IN INCHES:
26 x 1 3/8 x 1 1/4 (32 x 590)
The first number (26) designates, in inches, the outside diameter of the tire.
The second number (1 3/8) designates, in inches and fractions of inches, the
actual section width of the tire.
The third number (1 1/4) designates, in inches and fractions of inches, the
actual section width of the original tire for this rim.
MARKINGS IN MILLIMETERS:
650 x 32A (32 x 590)
The first number (650) designates, in millimeters, the outside diameter of the
original tire for this rim.
The second number (32) designates, in millimeters, the width of the tire.
The letter "A" corresponds to the 590mm rim.
SPECIAL CASES IN INCH MARKINGS:
28 x 1 5/8 x 1 1/4 x 1 1/8
700 C Course (32 x 622)
The first number (28) designates, in inches, the outside diameter of the
original tire for this rim.
The second number (1 5/8) designates, in inches and fractions of inches, the
height of the original tire.
The third number (1 1/4) designates, in inches and fractions of inches, the
width of the tire.
The fourth number (1 1/8) designates, in inches and fractions of inches, the
width of the rim that fits this tire.
