Here's an article on guages- it's accepted that a couple of pounds one way or another won't hurt although once it gets this way I correct it. Certainly 12% is dangerous and if you drive very long like that your tire will either fail or become compromised, particularly within the sidewall. You can also see other psts by Roger Marble by looking in the right hand margin. There's lots of data there-
If you are reading this blog you know that along with others, I constantly complain that people do not always maintain their tire pressure.
One part of the process is using a sufficiently accurate gauge. I consider +/- 2 to 3 percent or +/- 1psi when measuring passenger tires to be sufficiently accurate.
Over the past few years I have checked more than 40 different gauges and have observed a 14 to 18 percent failure rate in two groups of 20 or so gauges.
I have two digital "Master " gauges as seen to the right, which read to 0.5psi increment. Both have been checked against ISO certified laboratory gauges and one reads 0.5 psi high and the other 0.5 low. I compare these gauges to each other to confirm they have not gone out of calibration before I conduct my gauge test. I figure the chance of both going out of calibration the same amount and same direction at the same time is very unlikely.
This year while at the Gypsy Journal Rally in Celina, OH, at my Tire Basics seminar, I offered to check gauges. Here are the results.
Actual Indicated Type Gauge Error
82.0 81 Dial -1%
70.0 69 Dial -1%
77.0 75 Stick -2.6%
76.0 81 Dial +6.6% Fail
77.0 92 Short Stick +19.5% Fail
74.5 81 Long Stick +8.2% Fail
79.5 78 Long Stick -1.8%
77.5 78 Long Stick +0.6%
68.5 72 Long Stick +5.1% Fail
68.0 68 Dial =
68.5 74 Long Stick +8%
As you can see we observed a 36 percent failure rate with all the "fail" gauges giving a high reading which could result in the operator unknowingly operating their tires in an under-inflated state yet thinking the tires were properly inflated. Given the general tendency for RVs to be overloaded in the first place or to be operated right at the limit of the load capability for the specified inflation this is definitely not the better way for a gauge to be wrong.
Having your own accurate "master gauge" is not unreasonable given the low cost of good quality digital gauges. You don't even need a special dual foot master gauge.
Do your own "calibration check" by using the "master single head gauge to check a front tire. Then use your "truck" dual foot gauge (as seen on the left in the top picture above ) to get a reading on the same tire to confirm the reading. If the readings match you can be confident all your readings will be sufficiently accurate when you use your dual foot gauge on the rear tires.
I would suggest you get a digital gauge and then visit the tire store from one of the major tire companies and ask if they would check your digital against their master certified gauge. You could also ask when their master was last calibrated. A competent certification includes affixing a dated calibration sticker to a master gauge. If they cant tell you when it was calibrated just say "thank you" and go to a more competent tire dealer.
Finally don't just throw your master gauge in the bottom of your tool box. I suggest you keep it in a box of some type. Even a Tupperwear type container can protect your master gauge.
Also don't forget to always use a cap or screw on TPM sensor to keep dirt out of the valve core area of your valves. If you don't understand why you need a cap on the valve I suggest you subscribe to this blog as I will be posting a story showing exactly why you should always protect the valve core.