The tire size designated as 26 x 1.75 is common to many adult-sized mountain bikes and cruiser bikes. If you cannot find the size you need or perhaps want to experiment with other types of tires, one thing you must remember is that decimal tires and fractional tires are not the same size. That is, a 26 x 1 3/4-inch tire will not fit a 26 x 1.75 rim.
26 x Decimal
Although "fractional" sizes and "decimal" sizes cannot be substituted for each other, a wide variety of decimal tires will fit the same rim. Technically speaking, the 26 x 1.75 rim can handle virtually any tire from 26 x 1 to 26 x 2.125 in width. If you choose to use tires that are a different width, make sure the bike's frame and fenders provide enough clearance for the tires once they are mounted. You will also want to avoid tires that are too much different in width from the rims. You risk damaging the rims if the tires are too narrow, and the tire sidewalls if the tires are too wide.
The International Organization for Standardization, also called the ISO, uses an official tire sizing system to eliminate much of the confusion. In this system, sizes are measured by the diameter of the tyre's bead where it seats into the rim, which measures 559mm in the case of a 26 x 1.75 rim. As a general rule, any ISO system bike tire with the number 559 will fit a 26 x 1.75 rim.
ISO-ETRTO Metric 47-559
The European Tyre and Rim Technical Organization, or ETRTO, created a tire sizing system that was adopted by the ISO. An ETRTO measure gives the width of the tire in millimetres, followed by the diameter of the tyre's bead seat in millimetres. A 26 x 1.75 tire is equivalent to a 47-559 ISO-ETRTO tire, but a range of different tires may also fit the rim. Tires from 25-559 to 57-559 correspond to the 26 x 1 to 26 x 2.125 widths.
French Metric 650 x 45
French tire manufacturers have their own sizing conventions that pre-date the ISO standards. These typically combine the diameter of the tire in millimetres with a letter that represents the width of the tire. Perhaps because the width of these tires fit between the standard letters, the French equivalent of a 26 x 1.75 tire is 650 x 45. Remember that the 650 is the diameter of the tire, not just the tire bead.
Bead Seat Circumference 1756
On occasion you may come across tires sized according to the actual circumference of the rim. This measurement is called the "Bead Seat Circumference" (BSC) or "Bead Seat Diameter" (BSD) and is given in millimetres. A 26 x 1.75 tire has a BSC of 1756mm. Some manufacturers may list the BSC as 1755mm, but the two are the same.