What Eugene said is spot on. Sometimes hiring an attorney on the front end before any charges are filed can save you from going to jail on the back end. With that being said, if you can't afford a lawyer, keep in mind that you have absolutely no obligation to speak with this detective. They literally WILL use anything against you that you tell them. I would be very wary of going to speak with this detective.See question
Based on the information you have given, it sounds like the state/city has a weak case. You can file a motion to suppress and dismiss for lack of particularized suspicion. Under Montana Code Annotated section 46-5-401, an officer can stop a vehicle if the officer believes "the person or occupant of the vehicle has committed, is committing, or is about to commit an offense."
If the officer has no way of knowing what your speed was in the form of clocking you or otherwise gauging your speed through some recognized means, it will be difficult for the government to establish the basis of the officer's stop. Without a basis for the stop (i.e. an illegal seizure under the U.S. and Montana Constitutions has occurred), anything that comes after an illegal stop should be suppressed. All they have right now is the word of the officer, who has no explanation as to how he arrived at the conclusion you were speeding besides speculation and conjecture. Based on the limited facts you have given, this case has a better chance at being tossed than most.See question