Had the same problem with mine last year. Checked rpms at 4 different stages with a digital meter while going down the river. 1500,2500, 3500 and 4500 rpm. Found that in my boat the sync gauge was right. The old faria tachs were off. Got new gaffrig gauges just have not installed yet.

Wally might be right. It happened to me. Only 25 hrs on the boat. Props looked brand new. Had same problem, not quite the difference though, 4950 vs 4840 rpm or so. Had the props labbed and he reported back that one was a little out of round. We are going to test this weekend and see if the redone props help.

Good Luck.