Sturmey-Archer’s A2 puts two gears inside the rear hub and shifts automatically. At a standstill it is in low gear; accelerating past a set speed brings an upshift. Slowing down brings it back into low gear.
The A2’s stated range is 137%: the high gear is 37% taller than the low gear. Versions are offered with higher-speed or lower-speed switching, so the model name alone does not describe how a particular bicycle will behave.
The A2 and A2C also differ in braking. The A2 is the freewheel version, while the A2C incorporates a coaster brake. Shifting and braking are separate functions: automatic gear selection does not tell you how the bicycle stops.
Before choosing, ride the bicycle on your usual route. Is low gear easy enough for starts and climbs, and does the shift arrive at a comfortable pedalling rhythm? Two gears keep the choice simple, but offer fewer ways to adjust the effort than a wider-range system.