Have you requested the officer's report? That will be considered by the judge as the primary piece of evidence along with the radar certification, etc. The citation, for the most part, is simply a charging document but still must comply with some requirements. But it is the officer's report you must obtained by following the proper discovery rules. You can give it a try on your own in court but you are rolling the dice.
As Mr. Huff said, you really need to get the police report. Maybe the officer's report indicates that you were speeding in the 25, and that by the time the officer pulled you over your vehicle had already entered the 35 zone.
Also, if you are asked any questions about the incident you could be faced with the choice of either presenting evidence against yourself (not a criminal case so no 5th amendment rights here) or to lie to the judge (which is a crime).
I recommend hiring an attorney. The attorney will most likely be able to attend the hearing on your behalf (hard for the judge or prosecutor to ask you questions if you aren't in court!), they will probably be able to make legal motions in addition to the issue you raise about the 35 vs. 25 mph speed limit.
If you have any questions give me a call and I would be willing to give you some advice, and certainly more than willing to represent you if you want to talk about hiring me.
I agree with both previous answers and yes you should contest and your chances are better with an attorney to show the street signs and ask you the questions you need to show there is a problem with the citation.
Only If and until you and I sign an Agreement for Legal Services, I am not your attorney. These answers are provided for informational and/or novelty purposes