Bicycle tires come in many sizes, and each size is generally related to a particular type of riding. Mountain bikes, for instance, have very large tires made for off-road hazards and muck, while road and racing tires are thin, smooth and tailored for fast riding over smooth ground. Fitness and cross bikes often merge these two extremes and allow for slightly larger tires than would normally be seen on a road machine.
29x2.25: Mountain Bike Tires
The 29-inch by 2.25-inch mountain bike tire, or 29er, is a beefy, large-wheel tire intended for big races and fast riding. The large wheel diameter of a 29er-equipped bike allows for lesser rolling resistance and better tracking over the smaller, standard 26-inch tires. The big wheels are, however, less maneuverable and somewhat poorly suited to big obstacles and tight racing environments. Their main appeal lies in long, hilly rides where comfort and endurance are paramount to responsive bike feel.
700x28: Road Tires
The road tire standard is 700mm by 23mm, but the larger 700mm by 28cm tire adds stability and ruggedness to the somewhat flimsy road bike tire. The extra 1/2cm of tire width allows for greater traction on wet or crumbly pavement, but it also adds rolling resistance and changes bike handling. Some riders, particularly those new to road biking, prefer the forgiving cornering abilities and stable tracking of wider tires. Those who require minute response, however, are more likely to choose a smaller tire.
700x45: Touring and Cross Tires
The 700mm by 45cm tire wears several labels. Touring bikes use this hybrid-sized setup to offer a comfortable, easy-to-handle ride for those who just want to enjoy an afternoon on the bike path. Cyclocross racers, however might choose a beefy, technical tire sized to 700mm by 45cm for extra rugged or slick tracks where stability is favored over rolling resistance. Aside from rolling resistance, the obvious drawbacks to these tires is weight. They are unlikely to be chosen for long, hilly rides.