As an ex--fireman for the Chicago, Burlington and Quincy Railroad on the head locomotives of very high speed passenger trains such as the California Zephyr as a young man, trains sometimes running over 100 mph(speed limit 87 mph) we were always aware of our speeds.

Using either our RR certified pocket watches(or just introduced in the early 60's, Bulova Acutron wristwatches), we(Engineer or Fireman) were always train speed aware, and often would time our trains using mileposts and watches to check out the accuracy of the diesel locomotive speedometers, as accuracy was paramount.

Using this primitive technology, we would be close enough for government work,,,however if eyeballing your watch or the mile post you always were subject to a 'fudge' factor of several mph being common.

Using modern technology utilizing GPS either via a smart phone app or a Garmin type navigation device you are dead nuts on,,,,and far simpler. Calculate the answers to my 3 questions on mph and get back to me for further information, most who attempt will not have the correct answers.

It is impossible to accurately determine looking at an odometer that when the 10th mile indicator is exactly the same as it was when you started you measurement, however 5 or 10 mile measurement average will dial it in more accurately.


My point being that a GPS device is mandatory if you wish indisputable accuracy, if government work standards are sufficient and you have the time,,,mileposts and wristwatches will certainly do the job.

As for me I would prefer to have purchased and installed the correct speedometer gear on the first attempt.

The reason for suggesting to OP that a GPS device may help him in his quest.

The purpose of this forum is to help our fellow members in offering up opinions,,,,as to the most expeditious solution to problem that is presented

When I proposed a smart phone app to perhaps help him out, never occurred to me thst one should first ax if he had access to a smart phone.

Tough Crowd!