well, you have 2 choices:
1) To challenge the evidence from OPP, i.e. show me the record of when I speed
OR
2) To pledge for lower fine, since this is your first speeding offence
The best is not to exceed the speed limit by 18km/h on a highway. To avoid getting caught, the trick is not to exceed the speed limit by 12-15km/h on a long curve, where police could be waiting for you at the other end of it.