How Do You Check a Tire Inflator's Accuracy?
2 lessons in this series, 6 min in total
- How Accurate Are Portable Tire Inflator Gauges? 3 min read
- How Do You Check a Tire Inflator's Accuracy? 3 min read
This lesson draws on Ashcroft’s guide to gauge grades, JACO’s gauge listing, the Kia and AstroAI CZK-3674 manuals, and Reviewed’s inflator review. More on how we research.
What counts as a good reference gauge
Look for a published ASME accuracy grade. Gauge maker Ashcroft says the grade is a percentage of span (the full scale), and some grades are tighter in the middle half. Mechanical gauges are typically most accurate mid-range.
| ASME grade (per Ashcroft) | Allowed error, % of span | On a 0-60 PSI (0-4.1 bar) gauge, middle half |
|---|---|---|
| 2A | ±0.5% across the dial | ±0.3 PSI (±0.02 bar) |
| 1A | ±1% across the dial | ±0.6 PSI (±0.04 bar) |
| A | ±2% ends / ±1% middle | ±0.6 PSI (±0.04 bar) |
| B | ±3% ends / ±2% middle | ±1.2 PSI (±0.08 bar) |
The last column is the percentage times 60 PSI (4.1 bar). The same Grade A on a 0-160 PSI (0-11.0 bar) gauge allows ±1.6 PSI (±0.11 bar) in the middle half, so for 30-40 PSI (2.1-2.8 bar) tires, a 0-60 PSI (0-4.1 bar) gauge reads mid-scale.
JACO says its Elite 60 PSI (4.1 bar) dial gauge is calibrated to ±1.5% at mid-scale and certified to ANSI B40.1. By our math, that’s about ±0.9 PSI (±0.06 bar).
Kia’s manual says to check monthly or more with a good quality gauge, so buy or borrow one.
The 8-step check
- Start with cold tires. Kia defines cold as parked at least three hours, or driven no more than 1 mile (1.6 km).
- Read one tire with the reference gauge and write the number down.
- Set the inflator 2-3 PSI (0.14-0.21 bar) above that reading. The AstroAI CZK-3674 manual’s troubleshooting says the unit won’t inflate if the preset is below the current pressure.
- Let it run until it shuts off, and note what the display shows.
- Remove the chuck and read the tire again with the reference gauge.
- Compare your target with that reading. The gap combines sensor error, overshoot (air added past the preset) and chuck loss (air lost as the connector comes off).
- Repeat on two or three tires. If the gap stays about the same, shift your target by it, as Reviewed did with the Viair 85P: set 40 PSI (2.8 bar) to get 35 PSI (2.4 bar).
- If the gap jumps around, stop trusting the display. Use the inflator as a pump and the reference gauge as the gauge.
Run the check at the pressure you actually use, since gauge error can vary across the scale.
Why the check needs cold tires
Driving heat raises the reading. Kia’s manual says warm tires normally read 4 to 6 PSI (0.28-0.41 bar) above the recommended cold pressure. It also warns not to let air out of warm tires to match the label, or they’ll be underinflated once they cool.
Air temperature counts too: a check done in a warm garage won’t match the same tire on a cold morning, so compare at the temperature you’ll drive in (see how much a cold morning lowers tire pressure). Your door label lists cold pressures, so the reading you hold against it should be a cold one as well.
Using your offset (and fixing overshoot)
Once the gap repeats, write it on a strip of tape on the inflator, for example “set +1.” Add it to your target each time you inflate.
If a tire ends up high, let some air out and check again. Kia’s manual says to release air by pushing the metal stem in the center of the tire valve, then recheck.
If your inflator says 35 PSI (2.4 bar) and your gauge says 33 PSI (2.3 bar), go by the reference gauge. The gap can come from sensor error, a reading taken while air was still flowing, or air lost at the chuck, as covered in why inflator displays drift.
With your offset known, set each tire to the figure on your door label, and if you’re unsure which number applies, start with what PSI your tires should be.
