The only speedometer calibration which will stand up in court is one which is done by a certified mechanic. A year or two ago there was a case in another state where it was ruled that GPS is "new" technology which has not been proven to be accurate. The person testifying as an expert witness in that case was a paid employee of one of the police lidar gun manufacturers. To make a long story short, basically the dude committed perjury on the stand so that the accuracy of lidar guns would not be called into question -- even though it is well known that lidar guns can easily and deliberately be made to display inaccurate speed readings by panning or sweeping the lidar gun's laser beam across a surface, that surface being a vehicle, the roadway, or any stationary object which has a flat and evenly illuminated surface. Just yesterday, I clocked a neighbor's house moving at around -10 to +10 mph depending on what direction I chose to pan across the side wall of the house. This pan error, by the way, was done using the newest TrueSpeed model lidar gun.
Radar guns emit a much wider cone of energy, but at close distances that cone is a pretty small size. It is quite possible, on a sharp curve, that the state patrol's radar did not "see" your car until both of your cars were very close to each other. In this situation, the radar gun's beam is small enough to "pan" across the front of your car as both of your vehicles round the curve at the same time. In this situation, regardless of which car was on the inside of the curve, the radar gun would have generated a falsely higher speed reading.
Obviously your situation must have been as follows: it was a two-lane county road, it must have been a fairly sharp curve, and both of your vehicles encountered each other while both vehicles were rounding the curve. Am I right?
In court, don't question the officer's credibility. That will get you nowhere since the officer is also a sworn officer of the court. Instead you have to question his expertise in properly operating a radar gun. He doesn't have to understand the underlying principals of a radar gun's operation, but he does have to have had sufficient training and must possess knowledge of the various types situations which can cause any radar gun to display a false speed.
Most importantly, I suspect that the officer hit you with instant-on radar. This is a mode of radar gun operation where the officer releases a trigger button which makes the radar gun suddenly start transmitting. The officer then will usually kill the radar gun's transmission as soon as he obtains a speed reading. The problem with really short bursts of I/O radar is that the officer only hears a tone from the radar gun for less than 1 or 2 seconds. That is a pretty short time period in which to try to carefully listen in order to discern whether or not the radar gun is emitting a steady audio tone. If the tone isn't steady, then the speed reading must be considered suspect. Not only that, if the officer only used a single and very short burst of I/O, then the officer has no radar gun tracking history to match up with his visual tracking history.
Others either on this forum or on the radardetector.net forum can further elaborate on all of this information. You might want to copy your post and my reply over to the other forum in order to get additional information, comments, and opinions.