At first glance, it appears that the difference between a 26-inch wheel and a 28-inch wheel is just 2 inches of diameter. However, the difference in size is rarely exactly 2 inches, due to varying widths and depths of the tires used. In addition, the intended purposes, wheel material and bicycle style of these wheels vary significantly.
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Most bike frames fit only one size of wheel, or two very similar sizes of wheels. The 26-inch wheels and 28-inch wheels are dissimilar enough that you should not try to switch one for the other. The 28-inch wheels are too large to fit in a frame for 26-inch wheels, while the brake assembly on a bike using 28-inch wheels will not reach far enough to work with 26-inch wheels.
In the U.S., 26-inch wheels with decimal widths, such as 1.75 inches, appear primarily on adult mountain bikes, with some comfort, hybrid and cruiser bikes also using this size. Other types of 26-inch wheels were used on older Schwinns, English roadsters and French bicycles. The 28-inch wheels that are 1 1/2 inches wide appear on English, Dutch, Chinese and Indian rod-brake roadsters, and may also be called F10, F25 or 700B. Tires measuring 28 x 1 5/8 x 1 1/4 inches are a Northern European designation for 700C tires. Some German companies designate 700C tires as 28-inch tires with decimal widths.
Not all 26-inch wheels or 28-inch wheels are equal in size. The 26-inch wheels with a width measured in fractions of an inch, such as 26 x 1 3/4 inches, are actually slightly larger in diameter than wheels with widths measured by decimals, like 26 x 1.75. Confusingly, some, but not all, 28-inch wheels with fractional measurements are the same diameter as those with decimal measurements. Furthermore, many 28-inch wheels are actually the same diameter as wheels marked 29-inch, which are in turn the same size as the metric measurement 700C.
The 26-inch wheels with off-road tires will be slower than 28-inch wheels, because 28-inch wheels are used on roadsters and road bikes. Road tires, whether for 26-inch or 28-inch wheels, have less tread than mountain bike tires or even no tread, and therefore have roll faster on paved roads. However, 26-inch mountain bike tires have better traction in off-road conditions, which usually makes them faster for that use.
Steel is the standard wheel material for bikes made before the 1980s. However, aluminum wheels are the standard of the early 21st century. Fractional 26-inch wheels are an older style, not sold except as second-hand parts or built except for custom orders, and are steel exclusively. Most 28-inch wheels are steel and though uncommon in the U.S., they are still common on roadsters in Asia and the Netherlands. However, wheels with identical diameters to some 28-inch wheels, the 29-inch mountain bike wheel and 700C road wheel, are typically aluminum.