The cheap floor pump you've had for three years is probably reading 4–6 psi high. Bourdon tube gauges drift with age, and hose volume error adds systematic error on every use. A one-time calibration against a quality digital gauge takes 10 minutes and fixes your pressure setting permanently.
Why pumps read incorrectly
The gauge in a standard floor pump uses a Bourdon tube — a curved hollow tube that straightens under pressure and moves a mechanical needle. These gauges are inherently imprecise. They're designed to give an approximate reading in a cost-effective way, not to provide laboratory accuracy. New pumps from reputable brands (Lezyne, Topeak, Park Tool) are typically accurate to ±2–3 psi. After several years of use, drift of 4–8 psi is common.
Three mechanisms cause drift: mechanical wear on the Bourdon tube connection, which changes the calibration point; spring fatigue in the indicator mechanism; and accumulated contamination in the gauge from moisture and particulates. The drift is almost always in one direction — the gauge reads higher than actual. This is why riders who religiously pump to "80 psi" often find their tyres at 70–74 psi when checked with a separate reference gauge.
Hose volume error is a separate and less understood source of error. When you attach a pump head to a valve and the pump is at rest, the hose contains air at ambient pressure. The first few strokes equalise this hose volume with the tyre. On a high-volume tyre (40+ mm), the hose volume is negligible. On a narrow road tyre at 90+ psi, the hose volume represents enough air that the gauge reading when attached can be 2–3 psi lower than the tyre's actual pressure — which the pump then over-corrects.
The two-step calibration process
You need a reference gauge — a quality digital gauge like the Topeak SmartGauge D2X or the Silca Bluetooth gauge. These have MEMS sensors that are accurate to ±0.5 psi and don't drift the way mechanical gauges do. A dedicated digital gauge costs £20–60 and pays for itself in better tyre performance.
Step 1: Pump a tyre to what your floor pump gauge reads as a round number (say 70 psi). Record that number.
Step 2: Remove the pump, attach the digital reference gauge, and read the actual pressure. Write it down.
The difference is your pump's calibration error. If your pump said 70 psi and the reference says 65 psi, your pump reads 5 psi high. Add this offset to every future pump reading — when you want 70 psi, pump until the gauge says 75 psi.
Do this at two points in your pressure range — once at a low pressure (around 40 psi) and once at a high pressure (around 80 psi). Bourdon tube error is not perfectly linear; you may find the error is 3 psi at 40 psi and 6 psi at 80 psi. Average the two corrections or use the high-pressure correction for road and the low-pressure correction for MTB/gravel.
Maintaining calibration over time
Once calibrated, write the correction factor directly on the pump barrel with a permanent marker. A small label that says "+5 psi" is more useful than a mental note that decays after two weeks.
Re-calibrate once a year or after the pump has been used heavily (500+ pump sessions). A pump used for two bikes every week will drift faster than one used occasionally. Budget time for a quick re-check at the start of each season.
Temperature affects tyre pressure — 1 psi per 5°C temperature change, approximately. If you inflate in a warm garage (20°C) and ride in near-freezing temperatures (0°C), you'll lose 4 psi before you've turned a wheel. Some riders compensate by adding 2–3 psi above target for cold-weather rides. This is worth building into your pre-ride checklist rather than trying to calculate each time.
Altitude matters for very long climbs. Every 1,000 m of altitude gain is approximately 0.1 bar (1.5 psi) of ambient pressure change. For typical road cycling this is irrelevant. For riders who drive to high-altitude trailheads and inflate there before descending, check pressure at ride altitude rather than in the car at low elevation.