The Stopwatch is an informational accessory that displays the player's current movement speed. It does not require equipping in an accessory slot, and will work from within the player's inventory. It is purchased from the Traveling Merchant for 5 .
Speed is displayed as MPH, with 1 MPH being roughly equal to 0.75 tiles per second. For example: The Mechanical Cart's top speed is 102 mph, which equals 75 tiles per second (20 pixels per tick, with one tick being 1/60th of a second).
When running or in a Minecart, the Stopwatch's display only represent horizontal speed. When midair (jumping, flying, etc.), the displayed value is the total velocity in any direction.