Won't work. They will tell you (and rightfully so) that you should watch your speed and drive safely regarding of any changes on the car. If your speedo were off, you might have a chance. Can you do defensive driving? You will get an insurance discount too.
You can hire an attorney, but it is likely you will get deferred adjudication. That means you pay the fines and don't get another ticket for 60, 90, or 180 days. If you do, you are hosed. You will also pay your attorney.
Neither is cheap, but those are pretty much your options.
You can go to court and represent yourself trying to fight, but the odds are not great and if you lose, you could lose big.