The tire pressure myth most cyclists still believe
For decades, road cyclists pumped tires to the maximum printed on the sidewall on the theory that harder equals faster. Then people started measuring, and the theory fell apart. On real roads — which have texture, seams, and imperfections — an over-inflated tire doesn't roll faster; it bounces. Every tiny vertical hop is energy stolen from forward motion, plus fatigue hammered into your hands and back. The fastest pressure is the one that lets the tire absorb the road instead of ricocheting off it.
This calculator applies the approach popularised by Silca's research: target roughly a 15% 'tire drop' — the amount the tire compresses under your weight — which testing has shown puts the contact patch in the sweet spot between rolling resistance, grip, and comfort.
Why every input matters
System weight comes first — and it's the total: you, the bike, bottles, and bags. A 60 kg rider and a 90 kg rider on identical tires need very different pressures to achieve the same 15% drop, which is why copying a friend's pressure never quite works. Tire width matters just as much, and in the opposite direction than intuition suggests: wider tires need less pressure, not more, because they spread the same load over a larger air volume. A 25 mm road tire might want 80 psi where a 40 mm gravel tire wants 35.
The front/rear split is the detail most calculators skip. Your weight doesn't sit evenly between the wheels — typically about 40% front, 60% rear on a road bike — so the calculator prescribes a lower front pressure. Equal pressures front and rear means the front is over-inflated, washing out grip exactly where you least want to lose it: in corners.
Surface and casing finish the picture: rougher surfaces reward lower pressures, tubeless setups safely run a few psi below tubed equivalents because there's no inner tube to pinch, and supple casings deform more efficiently than stiff ones.
Using the number in the real world
Treat the calculated pressure as your baseline, not scripture. Ride it for a week, then experiment 2–3 psi in each direction: if your hands buzz numb on chip-seal, drop a little; if the bike wallows in hard cornering, add a little. Rim-strike protection sets the floor — if you're bottoming out on square edges, you've gone too low for the terrain regardless of what any formula says. Hookless rims add their own ceiling, usually 72.5 psi, that overrides everything.
One habit worth building: check pressure before every ride, not every month. Tires lose a few psi per week through the rubber itself — tubeless setups more — and yesterday's perfect setup quietly becomes next month's sluggish one. For the car equivalent of this optimisation, the tire size calculator covers automotive fitment.