Tach/ rev limiter/ msd 6aln box ?

Have 6800 chip in, have not hit it all season but tach recall reads 6990. What should i trust Quickcar extreme tach or Msd with plug in chip? What i am really asking is has anyone had a problem with either?Before you say it of coarse im gonna try another chip.

start by putting a 3000 rpm chip in and do a easy rev and see if it hits the rev limiter

this will tell you if your box and wiring is working correctly