First of all, a good lawyer can beat a pacing, Vascar, or estimated speed ticket. All those involve lots of human error, which can make big differences in what speed you're ticketed for. If the cop ran 80 to catch you, there's no way he can tell how fast you were going by how rapidly he was running you down. That's a HUGE target for a good traffic lawyer to zero in on.
He just pulled that 55mph number out of the air.
That said, you were admittedly speeding. The cop knows you were speeding, too...but what he doesn't know is how fast you were going, and him just making up a number will at the very least set you up for getting it reduced.
I'd go back and check the signs to see if you missed one. Cops are human, he might be mistaken.
My experience in traffic court (and I had a lot back in my youth) was, it's a racket. All the DA usually wants if you plead not guilty is to offer you a lesser speed that you'll plead guilty to. (unless it's some blatant, terribly outrageous violation) So around here, if you went in and said "Mr/Ms. DA, no way was I running 55mph. Not guilty", they'll pull you over to the side before you're up and say "is there a possibility you were speeding at all, maybe at say, 35mph?" Since that's much less of a hit on your insurance, most people will agree to plead guilty to that.
Anyway, I suppose that depends on where this happened, and how rural the court system is. If it's some backwoods judge, you might be SOL.
If it's a real court system in a fairly populated area, I'd probably lawyer up and fight it.