Faster isn’t always better.

In cycling, leg speed is measured in revolutions per minute (RPM), and for each revolution the flywheel spins five times. Bike speed, however, is measured in miles per hour (MPH), and is based on wattage. The higher your wattage, the harder your body is working to spin the flywheel.
Many indoor cycling bikes have a computer to measure RPM. On a road or trail bike, unless you have a cyclo-computer with a cadence option, you’ll have to calculate the number in your head. To do it, count how many pedal strokes you complete over 15 seconds, then multiply the number by four.
If you’re training for endurance, a higher RPM isn’t always better. To achieve that goal, you should train at a low intensity 80 percent of the time. That means you’ll need to find an RPM that brings you between 60 and 78 percent of your maximum heart rate. (To calculate your max heart rate, subtract your age from 220.)
Meanwhile, those who are vying for fat loss or improved VO2 max are wise to prioritize HIIT workouts, increasing RPM by 5 to 10 in each interval. Or, stay between 85 and 100 RPM and increase the resistance after each rest period. Increasing your resistance will also up your wattage. 
Of course, speed is only one part of the fitness equation. Resistance and body positioning also impact how hard you have to work. While maintaining a given RPM, ramp up intensity by increasing resistance levels or standing out of the saddle.