Well I'm not familiar with the traffic court system in SC nor am I a lawyer of any sort but hopefully this helps.
The 1mph difference thing is a bit fishy and I certainly would be tempted to cry foul as well but based on my experiences in traffic court I don't think you'll get much sympathy from the judge so I wouldn't waste your time on it. The 1mph difference shouldn't put you in a higher fine/points bracket so in the end it really doesn't make much difference.
Instead what I would do is plead not guilty and get your day in court. Then before you go to court I would do two things:
1.) File a discovery request with the DA's office (see Help! I Got a Ticket!
2.) If it's a public highway you should try to obtain the speed survey records.
If you aren't already familiar, all states are required to do periodic speed surveys and set their speed limits to the 85th percentile of that survey or come up with a good reason not to. If you can catch them with out-of-date survey results or a speed limit that isn't set correctly you may get a dismissal out of that.
In addition you should do the following to prepare for court:
Remember your goal is to discredit the officer's testimony. Ask him/her questions that call into doubt the accuracy of the reading and their judgment during cross examination. Try to anticipate ahead of time what you think the officer might say to each of your questions (see Beating a Speeding Ticket in Court | Lawfirms.com
). Above all, keep a level head. If you get angry the judge is less likely to sympathize with you, present your case objectively and hope for the best.