A general rule of thumb for mechanical speedometers: Under 40 MPH, a little behind the actual speed. Look out that can get you some tickets. Over 40 MPH, a bit optimistic. You are not going as fast as it says you are. There is an adjustment for the spring tension. If you see a lot of slower than indicated you can increase the tension and make it better sort of. If you see a lot of faster than indicated you can reduce the tension. But to do this you need to pull the speedo out and take it apart. Put it back and try again over and over till you get it close. If you know it is behind when slow and ahead when fast just adjust your self it is easy sort of.
