"Does anybody know of MTHs smph is even accurate?"
Your'e not measuring the actual motion of the object relative to fixed objects. You're measuring the number of revolutions the flywheel made over a period of time with the assumption that these rotations were actual reflections of the loco moving the "correct" distance for that time span. It doesn't handle wheel slip on straight aways or binding on tighter radius curves. Some people have tried to use the record function to get a train to repeat a sequence over and over again e.g. stoping at an unloading platform. Over repeated tries, the train will "arrive" close but not exactly at the same point.
Most automotive speedometers have the same issue. Unless you have an INS system in your car or a very accurate GPS or active radar, you only know your approximate speed (in real mph or kph).
