Pay the fine and get over it
They have proof you were speeding whereas you have no way to prove that you weren't.
You may have thought you were only 1-2kms over but just a twitch of the throttle on a Skyline will have you increasing by 4kph without knowing it. I know you feel aggrieved but chances are 99% that you were speeding. They normally knock off a few kms so chances are you may have been doing more than they claim