Law enforcement sometimes use "pacing" as a speed enforcement tool. The basic idea is that they consistently drive a certain speed - which is at or above the speed limit and notice that the "alleged speeder" is either keeping pace or exceeding the pace.
The details are complicated and a police officer would know them much better than me. But basically
- They have to calibrate their speedometer - because if their speedometer is broken pacing is obviously worthless. They need to bring the calibration results to court.
Not having those calibration records for your speedometer means that many police departments will be reluctant to issue a speeding ticket to the other motorists.
- They rely on the fact that most state laws allow law enforcement leeway to exceed the posted limit. Not being a lawyer or a police officer, I do not know the exact circumstances, but if they were not allowed to slightly exceed the limit for pacing then logically every pacing enforcement should result in 2 tickets - one being for the officer.
If you can swear that your speedometer is good then they can use that evidence to write you a speeding ticket, because whatever allowances the law allows law enforcement for pacing are not granted to you.
I am sure they can overcome the calibration issue with regard to a ticket issued to you by your certification that the speedometer is correct. If they issued a ticket to me based on your certification that your speedometer was correct, I would call bullshit.
Talking to the police can only hurt you.