Below is a conclusion from a research paper on the International SportMed Journal site showing how difficult it is to confirm the optimal cycling cadence.
Conclusion
A vast body of literature has examined various factors that may influence the optimal pedal rate to adopt during a variety of cycling tasks. Despite this research, the cadence which maximises performance during cycling remains unclear. It is possible that much of the uncertainty surrounding optimal cadences could be due to methodological inconsistencies between studies. In particular, the term ‘optimal’ may be used to describe the most economical, powerful, fatigue-resisting or comfortable pedal rates. As a result, the cadence that results in the best possible performance during the variety of cycling tasks experienced by cyclists appears to be multifaceted. Consequently, future research exploring the best possible cadence to select during cycling should examine a number of factors (i.e. power, neuromuscular fatigue, efficiency, blood flow and comfort) that may be associated with maximising performance outcomes. In particular, the influence of training at various cadences on performance and physiological adaptations requires further examination. Based on previous research, it would appear that muscle force and neuromuscular fatigue might be reduced, and cycling power output maximised, with relatively high pedal rates (100-120rpm). However, such high pedal rates increase the metabolic cost of cycling, especially at low power outputs (≤ 200W). As a result, short duration sprint cycling performance might be optimised with the adoption of fast pedal rates (~120rpm). Due to the influence that fast pedal rates have been shown to impart on cycling mechanics, cycling efficiency and fatigue development, performance in longer duration events might be enhanced from use of slightly slower cadences (~90-100rpm). During ultra-endurance cycling, performance might be improved by using relatively low cadences (70-90rpm), since cycling economy is improved and energy demands are lowered. Future research examining a multitude of factors known to influence optimal cycling cadence (i.e. economy, power output and fatigue development) is needed to confirm these hypotheses
The Athlete Clinic 
When cyclists are allowed to ‘self-select’ a cadence during fitness tests, they seem to typically self-select a cadence that follows the suggestions in this article. Science and research are confirming what cyclist have been doing in practice.
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