Tire sizing can be confusing. The confusion stems from the fact that the bike industry is clinging to an outdated system where tire sizes are supposed to represent the diameter of the tire when installed on the rim and inflated. However, one 20 x 1 1/8" tire fits a 451mm diameter rim and actually measures close to 20" in diameter while another 20 x 1 1/8" tire fits a 406mm diameter rim and is more like 18" in diameter. Similar situations exist with 16", 24" and 26" tires.
The International Standard Organization (ISO) method of tire sizing has been developed to help eliminate the confusion. The ISO system uses a two-digit number followed by a dash followed by a three-digit number, as shown in the photo below. The two-digit number refers to the width of the tire in millimeters and the three-digit number refers to the rim diameter in millimeters. The two digit number is roughly the width of the tire when pressed flat and measured from bead to bead. The three-digit number is extremely helpful and tells you what rim diameter the tire will fit on. For example, if you have a 406mm rim you must use a 406mm-tire.
Common recumbent tires are described in the following paragraphs:
16" Tires: Sixteen-inch recumbent wheels use either 305mm or 349mm tires. The 305mm wheels use fatter tires such as 16x1.5" or 16x1.75" while 349mm wheels use the 16x1 3/8" tires.
17" Tires: The 17x1 1/4" Moulton tire previously used on Lightning's and Windcheetah's is a 369mm tire. To our knowledge no one is using this tire as stock equipment on recumbents today.
20" Tires: Twenty-inch recumbent wheels use either 406mm or 451mm tires. The 406mm wheels are the most common and can be fit with tires ranging from skinny to fat. Common tire sizes for 406mm wheels are 20x1 1/8", 20x1.35", 20x1.5", 20x1.75" and 20x2.0". 451mm wheels, on the other hand, are limited to using either a 20x1 1/8" or 20x1 3/8" tire.
24" Tires: Twenty-four inch wheels use either a fractional designation 520mm road tire or a decimal designation 507mm-mountain bike tire. The Vision Sabre used the 520mm size and Rans used to put them on the 24" rear wheel V-Rex model. Some older VRex's may have been converted to the 507mm size.
26" Tires: Twenty-six inch wheels use either a 559mm tire or 571mm tire. The 559mm rim is found on all 26" mountain bikes and virtually all 26" wheel recumbents. A huge variety of tires, ranging from skinny to fat and cheap to high quality are available to fit 559mm rims.
650c Tires: The 571mm wheel is the 26" road bike size commonly referred to as 650C. This wheel is used with frequency on performance recumbents. Tires that fit the 571mm rims are typically high quality and very narrow.
700c Tires: 700c tires fit a 622mm rim diameter and are used on most present day road bikes as well as several recumbents. Both Easy Racers and Lightning use this size for many of their rear wheels. 700c tires are available in widths ranging from narrow to medium.