Seems like the officer may have cut you a break as many moving violation tickets get reduced to 311(a). Be careful what you write on these sites as well. I know you want to contest this, but even in your question you admit to speeding. This is something that I recommend you sit down and discuss with an attorney so you know your rights and options.
You already admit speeding so you are best off paying the $100 fine and be on your way. This is even less of a fine than 45 Ina 40. Also, if you have a car that doesn't go over 45 mph then it is most likely broken, in disrepair and you should probably not be driving it. Driving a broken vehicle is unsafe to you and all other on the road.