Either you were paced by the officer’s car, that is, he followed you and noted your speed on his speedometer.
The officer used a radar type device to determine your speed
The officer used a laser device
The officer used some distance timing device to determine speed like Vascar
The Officer used his eyeball to estimate your speed (yes, it happens and it is easy to defeat)
This is a quickie, for you to get the idea. Take the first situation. The cop paced you with his car. He has given his testimony and now it is your turn to ask him questions. You ask him is to present documentation from a recognized speedometer shop as to the last time the speedometer in the car he used that day was calibrated.
Surprise, he can’t. Few police forces calibrate their speedometers. If a police force does, chances are the officer does not have the documents with him. No, they can’t go and get them to try you some other day. The trial is now and if the prosecution was unprepared, too bad. The prosecution rested its case when they handed the witness over to you. The evidence presented now has a fatal flaw. That is why you don't seek this in Discovery or Subpoena anything. That certificate goes to foundation of the Prosecutions evidence. It is there job to bring it and be prepared just in case you ask for it. Chances are that it des not even exist, but if it does by requesting it before trial alerts them to what you are up to. Then if they have it they will bring it. You don't want that. The exception is the CFR Defense.
You immediately ask for the case to be dismissed. Case dismissed.
You did not prove anything; you simply asked for documentation that the prosecution was unable to present. By asking for it, it became an essential eliminate of the prosecutions case against you. Do not attempt to prove that the officer did not read his speedometer correctly. You simply took away their assertion that the speedometer was accurate to a legal degree of certainty. You cast doubt as to the accuracy of the speedometer.
The Judge knows that the speedometer, even though not 100% accurate was at least close. Close counts in Horseshoes, Atom Bombs and hand grenades. The Judge knows you were going fast. However the charge is an exact figure based on readings from a device that the prosecution can no longer prove to be accurate to a legal degree of certainty.)
Fines are based on an exact figure. For instance, 11 to 15 MPH over the limit $75, 16 to 20, $125. If you open you mouth and say “I was not going that fast”, you opened the door to a change in the charge. The court can assume by your own words that you were going over the speed limit. The court can now find you guilty of speeding and impose a fine without a definite speed. The charge can and will be changed to an arbitrary figure. You opened the door to a change of charge, you admitted to a crime.
Remember when you plead not guilty, you plead not guilty to going 15 MPH over the speed limit. That is your plea, and 15 MPH over the limit is their charge.
OK, the same works for Radar but there are differences. I will get into Radar, other devices and more detail later. I only wanted you to get a general idea here.