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I have a new bike and got a good digital gauge. The tires seem full but the gauge is only showing 6 psi and it should be 40. I don’t want to pop the tire by putting too much air in. Why would it show so low when it’s definitely more than 6.

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  • 10
    Are you sure the gauge is not displaying Bar (6bar=90psi)?
    – mattnz
    Mar 10 at 0:53
  • 3
    If your pump is Schrader, try hooking it to a passenger car tyre as a test. They should be around 30-35 PSI
    – Criggie
    Mar 10 at 1:11

2 Answers 2

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I like the suggestion in the comments that infer you should check the scale mode of the digital gauge. Confirm the readout is in PSI and not Bars. If the scale is confirmed to be PSI and the readings seem unusually low for the feel of the tire, the error may be due to a partially blocked valve stem.

Bicycles with tubeless set-ups typically have what's called a Presta valve and stem. These are notable for their thin stem topped with a nut on top of the valve assembly that must first be loosened in order to add or remove air and to check the internal pressure. The use of sealant, many times added to the interior of the tire through the valve stem (the valve assembly is entirely removed from the top of the stem for this) in the tubeless systems, can congeal in a portion of the stem. This will affect air flow (especially outflow) through the stem and confound readings of a tire gauge.

Often times you can determine this is the case by loosening the Presta valve's sealing nut and pressing it down. Normally air is released from the tire with this maneuver. If you find air is not being released or the air flow isn't fairly vigorous from an inflated tire, a sealant blockage could be to blame. Sometimes this can be rectified by attaching a Presta compatible pump and briskly infusing some bursts of air into the tire as a way to clear the stem blockage.

Related to this (still dealing with Presta systems), is errors related to the testing procedure. The nut should be loosened (opened, not removed) fully to its stop, the end of the testing device must be Presta valve compatible, and it should seal the valve completely during testing. A quality tester can be held sealed on an open valve without air escaping except into the testing gauge. There will not be a continuous air leak in a valid test. In fact, it's a good technique to hold the tester in place for a few seconds to see if the reading remains consistent. A partially plugged stem can create a situation where it takes a few moments for the gauge to show the actual internal tire pressure.

What I suggest after the checks above is to experiment. Take a pressure reading and then add some air to the tire. Recheck the pressure reading. Obviously it should have gone up. Despite the 40psi rating of the tire, it can handle much, much more pressure. To burst an undamaged tire correctly mounted to a sound, compatible rim would take an air compressor with the ability to deliver well over a 100psi several seconds (closer to the minute mark than the 15 second mark) to do it. And using a hand pump to burst such a set-up--well that would be a good, "hold my beer" challenge. Anyway, vary the air pressure in the tire and check that you and the gauge can accurately determine the pressure changes.

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  • "an air compressor with the ability to deliver well over a 100psi several seconds" [citation needed]. Go to your local gas station and use their compressor to inflate your tires. It only takes seconds to burst a tire, not because of the massive pressure delivered by the compressor, but by the volume of air. Car tires generally run at 30-40 PSI, well below bike road tire ranges, but the car tire needs many orders of magnitude more volume of air at that lower pressure to support the weight. Trust me, I blew several tires as a kid when I didn't have a proper bike pump...
    – FreeMan
    Mar 13 at 18:23
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You do not specify the type of your valve and how do you take the reading. If it is Dunlop valve, you are not expected to see the correct reading in pauses between strokes, only immediately while pushing air into the tire.

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