From 70 to 90 pedal rpm is ideal for bicycling. It should be maintained uphill, downhill, on flatland, against a head wind or with a tail wind. Pedal rpm translates into cadence, and the goal is to keep this constant. Everyone has a different cadence. This is obvious when you observe people walking, running or pedaling in a group.
People move their bodies at different rates over varying distances.
Keeping your bike in high gear one with more resistance does not necessarily make you go faster. Instead, it can stress out your legs. And using all legs to go faster doesn't work because bicycling is essentially an aerobic activity. Strong heart and lungs are the key. Therefore, to increase speed, work up to a cadence at which you feel steady and comfortable for long periods of time.