This motorcycle has a 6-speed manual transmission.
I climbed the hill in third gear.
I shifted into third gear before the curve.
速 specifically counts gears or speeds in a transmission (5速), while 段 is a broader counter for steps, levels, or ranks that can also be used for gears in some contexts.
From the kanji 速 meaning 'speed', used as a suffix/counter for gears or transmission speeds.
speed per hour; hourly speed
high speed; rapidity; swift movement · rapid-service train; rapid train
deceleration; slowing down
speed of light
low speed; slow speed
wind speed
gear shifting; changing gears
stall; lose airspeed · downturn; slowdown; slump; decline
speed of sound; sonic speed
speed per second; velocity per second · instant; moment; a few seconds
express mail; special delivery
great speed; extreme swiftness
shorthand; stenography
pitch speed; speed of a pitched ball
double speed; 2X speed · -times speed; ...X speed
detonation velocity; VOD · extreme speed; explosive speed
initial velocity
fastball
speed; velocity
speedy effect; quick results; immediate action
rapid fire; quick firing