Could i fight this ticket?

I received a speeding ticket from a college cop the other day. It was my first one ever and I'm 22. She said I was going 32 in a 25. I had just stopped at a yield sign then she pulled me over so there was no way I could of sped up that fast and there were cars in front of me so for her to actually get me down the first street I was on would of be impossible from where she was sitting. Plus the street that I was driving down had been 35 since I have lived there and just like last week changed to 25 and I had no clue until she told me that this whole area is now 25. I know I can either plead not guilty, defensive driving or pay the bill. I just don't know which one. Do I have a good enough defense to plead not guilty. I had a penger in the car with me and he said that it was stupid and there was no way I could of been speeding that much if any.