I honestly didn't think i was speeding. how do i fight this?

i just got a speeding ticket in wayne county missouri. the officer was a state policeman. when he pulled me over he asked me why i didn't slow down and i told him i truly didnt think i was speeding. i was driving my husbands truck which i never drive. i was in an unfamiliar area. going downhill. lol i swear! i know it sounds absurd. he wasnt hearing any of it but it was all true! i looked at my speedometer when i saw him and was actually going around 58. he gave me a ticket for 67 in a 55. i never speed! please help. how do i fight this ticket