an interesting article I found in regards to Cadence :
Using a slightly lower gear with a faster cadence can be more efficient than riding a higher gear with a slower cadence. The difference in efficiency is due to the unique physiological demands of each style. A high cadence pedaling style involves frequent, low muscular force productions while a low cadence pedaling style is comprised of less frequent, but more forceful muscular contractions. Since the slower cadence requires more muscular force per each pedal stroke, a greater percentage of fast twitch muscle fibers are recruited. Fast twitch muscles are not as efficient in their use of glycogen, so your body’s energy resources are drained more quickly when they are used. This can lead to increased fatigue late into your rides. During a stage race or when training frequency is high this becomes very important since stresses are compounded from one day to the next. On the other hand, the relatively low muscular force used with a high cadence relies more heavily on the slow twitch fibers, which are more aerobically efficient in using the body’s energy stores to produce work.
Typically, a cyclist will be most efficient with a cadence of 80 to 90 revolutions per minute (RPM). Cadences lower than 80 RPM require greater muscular forces, while cadences over 100 RPM place a higher demand on the aerobic system. Incorporating low and high cadence workouts into your routine will have you best prepared for your goals, though a majority of your riding should be done with an average cadence of 80 to 90 RPM.