In the context of your question, a motorcycle's speed depends on two things: 1-RPM and 2-Gear. For each gear, the speed of the motorcycle is low at low RPM and high at high RPM. I have a Suzuki GS-150 (which has a tachometer showing RPM) and in 5th gear the speed is 12 kilometers per hour per 1,000 RPM. Which means that at 4,000 RPM the motorcycle speed is 48 km per hour, at 5,000 RPM the speed is 60 km per hour and at redline (10,000 RPM) the speed is 120 km per hour.
With the lower gears the speed is lower than in 5th gear at the same RPM. For example, in 2nd gear the speed is approximately half of the speed in 5th gear, the gear ratio for 5th gear is 0.913 and the gear ratio for 2nd gear is 1.857.