I have my GReddy BC and boost guage teed into the same line (balance tube) and I get different readings. I know that the guage is in mmHG and the BC is in psi but my guage and bc fluctuate. I have tested and found no boost leaks, but still replaced almost all of the vacuum hoses cause they were getting old. The only thing that I can think of is that the boost guage is liquid filled which could be causing the difference.