Deciding on a wheel size shouldn't be confusing. Most road bikes have 700c wheels, while the smaller 650c wheels are more common on mountain bikes. Smaller-frame road bikes are often equipped with 650c wheels, but some manufacturers sell small-framed bikes with the larger wheels. And there may be some circumstances when you might want to put the 650c wheels on a larger-framed road bike. It's easy to cut through the confusion when you understand the differences between 650c and 700c bike rims. While for many, the differences may not be critical, there are notable differences between the functionality and performance of 650c and 700c rims in certain circumstances.
Weight and Rotational Speed
An obvious difference between 650c and 700c tires is that the 650c tires are smaller in diameter and lighter by about 200 g. The importance of this 200 g difference is magnified by the fact that the diameter of the wheel creates a greater rotational mass. In other words, it takes more force to get the wheels moving when you have more mass on a larger-diameter wheel. Because of this, the smaller wheels have better acceleration capabilities and they climb hills better. Once you are at cruising speed, though, the greater momentum of the spinning, larger wheels render mass differences negligible to the rider. Accelerating quickly may make a difference in road races or in hilly terrain, but it should not matter much to casual riders or to those who participate in long-distance events where you maintain a constant speed.
Speed and Gearing
The supposition that bigger wheels are faster is not true. Several record-setting bikes have had small wheels, notes R&E Cycles. The wheel size does not affect speed. One revolution of the crank does result in less forward motion with small wheels, but your crank speed and cadence are determined by your gearing. With multispeed cassettes, you can easily adjust the gears to establish the cadence and crank speed.
Safety, Comfort and Stability
The larger wheels, with their wider radius and gentler curve, stabilize and smooth out the ride, especially on rough pavement. Larger wheels, especially when they are used on smaller frames, create more toe overlap though. If your toe hits your front wheel when you turn, you have toe overlap. Toe overlaps greater than 1 cm create the need to stop pedaling when you turn. You lose speed and stability, and if you pedal, you are at risk for an accident. Also, for commuters or others who want to use fenders, you often don't have the toe clearance to do it with the 700c tires.
The Best Fit for You
For most riders, the performance differences between the two wheel sizes probably evens out. Still, the rim size can affect the fit of the bike and the comfort of the ride, especially if you are tall or short. Riders less than 5 feet 7 inches generally find a better fit on a bike with a smaller frame. A too large frame and wheel can reduce your standover clearance. It can force you to reach for your handlebars and compromise your riding position, reducing power and causing discomfort. If you are taller than 6 feet, you may find that the smaller wheels cramp your riding position, so your knees bend too much and you have to bend too much at the waist to assume a horizontal racing position. Whether your focus is on leisure and comfort or speed and performance, your fit and positioning on the bike should be among the most important factors that determine what size wheel makes the most sense for you.