Which Pressure Gauge Do You Need?

How this is worked out

The rule

• target_mid_psi = (range_low + range_high) / 2
• required_full_scale = smallest standard scale in [30, 60, 75, 100, 150, 160, 250] that is at least range_high x 1.2
• target_band = target_mid_psi / required_full_scale // where the needle sits on the dial
• in_middle_half = 0.25 <= target_band <= 0.75
• applied_percent(grade) = in_middle_half ? grade.middle_half : grade.outer_quarters
• tolerance_psi = applied_percent / 100 x full_scale // a share of span, not of the reading
• error_at_target_percent = tolerance_psi / target_mid_psi x 100
• A Grade A gauge is called for when Grade B error at target exceeds 4 percent of the reading.

What it assumes

• Ranges are typical published ranges for the vehicle class, not your vehicle. The door placard governs.
• Grade A is taken as 1 percent of span over the middle half of the dial and 2 percent over the first and last quarters, and Grade B as 2 and 3 percent. That banded structure is how the gauge standard states accuracy; consumer listings usually quote only the middle-half figure.
• The 1.2 headroom factor keeps the reading off the top of the scale, and on every vehicle class here it also lands the target inside the middle half, where the tighter figure applies.

What it does not account for

• Gauge drift over time. A dropped dial gauge can read wrong and look fine.
• Temperature. Pressure read after driving is not cold pressure.
• Valve fit. A Presta valve needs a head that closes on it, whatever the scale says.
• Digital gauges quoted in fixed PSI rather than a percentage, which do not map onto the grade math.