How to pick tire pressure — the Silca/SRAM formula

Why pressure matters

A tire at the right pressure is faster, safer, and more comfortable. Over-inflate and grip disappears, especially in corners and over rough surfaces. Under-inflate and you bleed watts to rolling resistance, then catch a pinch flat.

There is no single correct number for everyone. The right answer depends on your weight, the bike's weight, the tire width, the surface, and whether you run a tube or tubeless.

The Silca / SRAM approach

Josh Poertner — formerly Zipp's chief engineer, now running Silca — published the Pro Tire Pressure Calculator that has become the de-facto industry standard. SRAM released their own guide in 2023 using essentially the same model.

The core idea is simple: pressure scales with per-wheel load and inversely with tire width to the power 1.4. One formula spans 23 mm road tires through 50 mm gravel rubber, no lookup tables required.

Why the rear runs higher

Weight distribution on a road bike is typically 40 % front / 60 % rear because the saddle sits behind the bottom bracket. On gravel the rider shifts slightly further back (58/42) to keep the rear wheel planted on loose surface.

Result: the rear wheel carries more load and wants roughly 5 psi more pressure than the front. Pump both tires equal and you under-inflate the rear, over-inflate the front, and lose both comfort and grip.

The corrections

	Condition	Adjustment	Reasoning
Wet road	−10%	Grip becomes more valuable than rolling speed; softer tire forgives a small slide
Hardpack gravel	−22%	The tire absorbs micro-vibration and saves the rider hours of buzz on long days
Loose gravel / rocks	−34%	Wider contact patch tracks instead of skipping across the surface
Latex tube vs butyl	−5%	Latex deforms more readily, run a touch lower without bottoming out on the rim
Tubeless setup	−12%	No tube to pinch — push pressure meaningfully lower for compliance and grip

What not to do

• Do not run sidewall maximum. «120 psi max» is a safety ceiling, not a recommendation. 90 % of amateur riders are 20–30 psi below max for everyday road.
• Do not trust a cheap floor-pump gauge. A 10 psi error is normal on a budget pump. Silca Pista, Lezyne Steel Floor, or a Topeak SmartGauge digital — accuracy pays back the first ride.
• Do not skip the pre-ride check. Tires lose 5–10 psi per week (latex faster than butyl). A 30-second check before every ride is the cheapest free speed there is.

What to do next

Once pressure is dialled, check that your tire width fits the frame. Most modern road frames clear 32–34 mm; gravel frames go to 45–50 mm. Look up the max tire clearance in the geometry guide of any frame in our catalog. A narrower tire at the right pressure outruns a wider one at the wrong pressure.

Sources: Silca Professional Pressure Calculator (Josh Poertner). SRAM Tyre Pressure Guide. Heine, J. (Bicycle Quarterly, 2019). «Wide Tires at Low Pressure» — empirically grounded research from the Compass/Rene Herse program.