Join Date: Mar 2008
Location: The 'Wack, BC, Canada
Bikes: Norco (2), Miyata, Canondale, Soma, Redline
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For the cost of the guages used and the acceptable tolerance there is no way to tell which, if any, are dead on accurate. It's unlikely but the one that is the farthest out and away from the grouping of the others just might be the most accurate. And as mentioned unless there's a 5 psi or greater difference they would all be considered to be OK by their makers because of the low cost of these items.
If you're feeling like you just have to know for sure you'd need to obtain a lab grade guage that comes with a calibration chart and that has a certified tolerance of something under 1%. But such things are not going to be cheap. Then your other guages would be tested against this certified guage and you'd print out a difference card that says what the offset is for each 5 or 10 psi jump. In use you'd go with the cheap guage but add or subtract the difference based on the chart for that guage.
Such a bother is the only way you can be certain that any guage you have is spot on or if it needs to carry a difference fudging chart.