Visual systems adapt to the prevailing image conditions. This improves the ability to discriminate between two similar stimuli but has the side effect that veridical perception is degraded. For example, prolonged driving at 100 km/h may reduce the perceived speed to 80 km/h but improve the sensitivity to changes in the prevailing speed. Here we use radially expanding flow fields with a wide combination of adapt and test speeds to study human speed perception. Adaptation at speeds higher than the test always attenuates perceived speed, whereas adaptation at low and testing at high speeds increases perceived speed. We show that adaptation is stronger (i.e., post-adaptation speeds are perceived as slower) when the dots in the expanding flow field accelerate towards the periphery rather than traveling at constant speeds. We also show that speed discriminability is reduced following adaptation to low speeds when tested at high speeds and increased when the test speed is at or below prior adaptation speeds. We conclude that the relative speeds of the adaptation and test patterns are important parameters governing speed-related adaptation effects in the human brain.