ANSI B40.1 Accuracy Grades

When a gauge is stated at 2 percent, that 2 percent is of the full span of the dial. On a 0 to 100 PSI gauge it is 2 PSI, and it is 2 PSI whether the needle is at 90 or at 20.

At 90 PSI that is a little over 2 percent of what you are reading, which is fine. At 20 PSI the same 2 PSI is 10 percent of your reading, which is not.

This is the whole argument for buying a gauge that fits your pressures rather than one that covers everything. A 250 PSI gauge used to set a 32 PSI car tire carries an allowance of several PSI at a reading where a single PSI matters.

For the lettered grades, the allowance is stated in bands across the dial. The middle half gets the tight figure and the first and last quarters get a figure one point wider.

Grade A is documented as 1 percent of span across the middle half and 2 percent in the outer quarters. Grade B is 2 percent in the middle half and 3 percent in the outer quarters.

That structure is why the gauge picker on this site keeps headroom above your working range: pushing the reading into the middle half of the dial buys you the tighter band. It is also why the figures quoted in consumer listings, which are almost always the single middle-half number, are the best case rather than the worst.

The numbered grades are stated as a single percentage of span across the whole dial: 4A at 0.1 percent, 3A at 0.25 percent, 2A at 0.5 percent and 1A at 1 percent. Those are laboratory and process instruments and you will not meet them on a tire gauge.

The lettered grades are the banded ones, and A and B are the two that turn up in consumer listings. The standard defines looser grades below B as well; this page does not reproduce them because nothing in the lineup here claims one.

One wrinkle in the naming. Listings on this site cite ANSI B40.1. That designation was consolidated into ASME B40.100, which now covers dial gauges, digital gauges and the related attachments in one document. A listing citing B40.1 is citing a superseded name, which is common and not in itself a red flag.

The first table below turns the percentages into PSI at every scale a gauge is commonly sold in. Read down it and the cost of buying a bigger scale than you need becomes obvious: the allowance is a fixed number of PSI, so a larger dial buys a larger error at your reading.

The second table runs the whole calculation for each vehicle class this site covers, showing the recommended scale, the pressure you are aiming at, and what a Grade B gauge is allowed to be wrong by there. That table is generated by the same code that runs the gauge picker, so the two cannot disagree.

The one class where the arithmetic demands a tighter grade is the mountain bicycle, because the target pressure is low enough that a Grade B allowance eats more than four percent of the reading.

Many digital gauges are quoted as plus or minus a fixed number of PSI rather than a percentage of span. That figure does not map onto the banded grades at all, and converting between them is not meaningful.

Compare digital gauges by their stated PSI figure directly against the PSI allowance you computed for a dial gauge. A digital gauge stated at plus or minus 1 PSI is doing better at a 32 PSI target than a Grade B 150 PSI dial gauge, which is allowed 3 PSI there.

Watch for the two other digital variables: the resolution of the display, which is how finely it reports and says nothing about accuracy, and the battery, which is a failure mode a dial gauge does not have.