Don't know about calibration, but these are electronic speedometers with electrically operated mechanical odometers, so they should be accurate internally. Mine does, however, have the gauge needle attached at the wrong angle, so it reads about 2 mph fast at all speeds. Sounds like you have the same problem, but the other way.
Removing and reinstalling the needle is a process though. The needle has a stop at 0, and the servo behind it does not. IIRC, you have to take the Speedo out, turn the needle to the right until it points straight down by itself, gently pull it off, press it back on at the correct angle (straight down, but adjusted from the angle it was at by the amount you noticed it being off), then you turn in back to the left until it rests against the 0 stop. There's also a high probability of breaking the needle in the process.
I just live with mine being off because I don't want to risk breaking it or making it worse.
Removing and reinstalling the needle is a process though. The needle has a stop at 0, and the servo behind it does not. IIRC, you have to take the Speedo out, turn the needle to the right until it points straight down by itself, gently pull it off, press it back on at the correct angle (straight down, but adjusted from the angle it was at by the amount you noticed it being off), then you turn in back to the left until it rests against the 0 stop. There's also a high probability of breaking the needle in the process.
I just live with mine being off because I don't want to risk breaking it or making it worse.
