I've seen a +/- 10 percent across my automotive tire pressure gauges. I'm enough of a weeny that I look for two that are very close and then able to adjust for the third. Not that I use all three, just my standard calibration method. I don't trust any of it.
But tire pressure is also sensitive to temperature, so I only check when they have been sitting overnight and the garage isn't baking hot. Roughly speaking, every 10 degree F change affects tire pressure by 1 psi. And if I drive from Wisconsin into Colorado, I don't stress that my tire pressure is suddenly reading higher. I tend to set my car tire pressures about 3 psi over the spec simply to allow for ambient temperature changes.
But 6 psi is a lot in steady-state conditions. I think I get a new gauge about every year as one always seems to get wonky every few years. The gauges on my pancake air compressor seem to be the worst. And I don't trust the stick style at all unless it's for very low pressure tires like on an ATV or lawn tractor, where 10 percent error doesn't really matter that much in the big picture.
Yes, I'm a tire pressure weeny. Is there a support group?
Eville Rich
2016 S10