Speeding Tickets: Ticketed for 68 MPH, But Speedometer and GPS Said I Was Doing 58

My question involves a traffic ticket from the state of: California, Motor officer was coming towards me with his lights on prior to this I looked at my speedometer and GPS they said 58mph got ticket for 68 in a 55, I was driving a 15000 lb truck on a road that was very windy and hilly small hills but some very sharp curves was doing mainly between 30 and 45 MPH stopping too let cars go by me due to slow speed what can I do it is his word against mine

Speeding Tickets: Ticketed for 68 MPH, But Speedometer and GPS Said I Was Doing 58