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Gravel tire pressure for 40mm tires, and what changes at 38 or 45mm

Published 24 September 2026

Disclosure: Pedal Brief may earn a commission if you buy through links on this page, at no extra cost to you. This version has no affiliate links. We have not run our own roll-down tests. The numbers come from our calculator's published method and the sources linked below. Full disclosures.

Who it's for: gravel riders on 38 to 45mm tires who want a sensible first number and a way to adjust from it.

Short answer: for about 80 kg of rider plus bike on 40mm tubeless tires and a 25mm internal rim, our tire pressure calculator gives about 33 / 35 psi (2.28 / 2.45 bar) on its gravel setting. Vittoria's figures for the same weight, as published by BikeRadar, are 31.9 / 33.6 psi. Adjust from there in 1 to 2 psi steps.

Start with load per wheel, not total weight

The most common mistake is treating the bike as one load. Frank Berto's method aims for a tire drop of "about 15% of W (the Section Width)." (Berto) Bicycle Quarterly, which republished his chart, is explicit that the weights on it are "wheel loads, not the weight of the entire bicycle." (Bicycle Quarterly)

The rear wheel carries more. Bicycle Quarterly's measured splits are 45% front and 55% rear for a randonneur bike, 40/60 for a racing bike and 35/65 for a city bike. (Bicycle Quarterly) Berto's rule for a rider on the drops was about 45/55, which he turned into roughly 10% less pressure at the front and 10% more at the rear. (Berto)

Our calculator uses a published curve fit of Berto's chart, P = 600·L/W² + 0.75·W − 25 (P in psi, L the wheel load in pounds, W the actual tire width in mm), then applies surface and setup corrections. BikeLab Studio documents the same fit. (BikeLab Studio) The calculator's method section lists every step.

Starting pressures for 40mm tires

Calculator: tubeless, 25mm internal rim, gravel setting, default split. Vittoria: figures for 40mm tires on mixed terrain, as printed by BikeRadar, based on what Vittoria calls "gravel endurance" riding. (BikeRadar) Front / rear, psi.

Rider + bike Pedal Brief calculator Vittoria via BikeRadar
60 kg (132 lb) 26 / 28 29.6 / 31.2
70 kg (154 lb) 29 / 32 30.8 / 32.4
80 kg (176 lb) 33 / 35 31.9 / 33.6
90 kg (198 lb) 37 / 39 34.2 / 36.0
100 kg (220 lb) 40 / 43 35.3 / 37.2

The two agree in the middle and part ways at the ends. Berto's approach holds tire drop constant, so pressure keeps rising with load. Vittoria's figures flatten out for heavier riders: its 100 kg and 110 kg rows are identical. (BikeRadar) If you are light, our numbers may feel soft. If you are heavy, they may feel firm. Both are reasonable places to start.

What changes at 38mm and 45mm

Width matters more than most riders expect, because the load term divides by width squared. Calculator output only, same settings:

Rider + bike 38mm on 23mm rim 45mm on 25mm rim
60 kg 27 / 30 25 / 26
70 kg 32 / 34 27 / 29
80 kg 36 / 38 30 / 32
90 kg 40 / 43 33 / 35
100 kg 44 / 47 36 / 39

An honest caveat: Berto's chart stops at 37mm. Everything wider is extrapolated from the curve fit, and the calculator says so. Vittoria's 45mm figure at 80 kg in the same BikeRadar piece is 30.1 / 31.7 psi, close to ours. (BikeRadar) That is reassuring, not proof.

Your real tire width is probably not 40mm

The formula wants the tire's actual inflated width. Vittoria gives the ETRTO rule of thumb: tire width changes by "±0.4 mm for every 1 mm difference from the design rim width." (Vittoria) Our calculator uses that rule and assumes a 23mm design rim for 35 to 46mm tires. That assumption is ours.

Measured gravel tires grow a little less. Bicycle Rolling Resistance put three gravel tires on 18, 22 and 26mm rims and found width rose "0.3 mm per mm increase in rim width." (Bicycle Rolling Resistance) Either way, a 40mm tire on a wide rim can measure 41mm, and on a narrow older rim it can come up under 40.

In the calculator, an 85 kg system on 40mm tires gives 36 / 39 psi on a 21mm rim and 35 / 37 psi on a 25mm rim. If you own calipers, measure the inflated tire and use the measured-width field. It beats any estimate.

Surface: how much lower for gravel?

Berto's chart describes pavement. Bicycle Quarterly says that on "very rough roads or unpaved roads, it may be useful to reduce the" pressure. (Bicycle Quarterly) Rene Herse: "On rough roads, lower pressures are faster." (Rene Herse)

How much lower is where published methods get vague. Our calculator applies:

Vittoria's terrain table moves surprisingly little. For 85 kg on 40mm, hardpack is 33.7 / 35.5 psi and mud is 32.0 / 33.7 psi. BikeRadar also suggests dropping pressure slightly when it is wet. (BikeRadar)

Front versus rear

Apply Berto's static 45/55 split literally and the front ends up noticeably softer than the rear. Some riders like that. Others find the front vague under braking, when it carries far more than 45% of the load.

BikeLab's calibration against SILCA and Rene Herse settled on a front pressure of 93% of the rear, and that is our calculator's default. (BikeLab Studio) Vittoria's figures run about 5% apart front to rear. You can switch the calculator to Berto's 45/55 or 40/60 split, and 40/60 suits a bike with bags on the back.

Tubeless or tubes

With tubes, the low limit is usually the pinch flat. Berto called it a "snake bite." (Berto) BikeRadar notes that without tubes "you can run lower pressures with a lesser chance of pinch-flatting." (BikeRadar) Our calculator adds 5% when you choose tubes, following BikeLab, which found the older +10% overshot in its validation. (BikeLab Studio)

Tubeless has its own low-end problem: burping, where the bead briefly unseats and loses air. If it happens on hard hits, add air.

Hookless gravel rims

The 5 bar road ceiling rarely matters on gravel, but width-specific limits are lower: 3.5 bar (51 psi) for 40 to 44mm tires and 3.0 bar (44 psi) for 45 to 54mm. Those are Schwalbe's figures (Schwalbe), and they match the ISO table of straight-side rim pressures that Bicycling reproduces (Bicycling). Bicycling's article text says 40mm tires "should not exceed 43.5 psi," but its own reproduced table gives 43.5 psi (300 kPa) for 45 to 54mm tires, not 40mm. We use the table. Heavier riders with bikepacking loads can get close. With hookless selected, the calculator warns above these figures for 35mm and wider, and your rim or tire maker's lower limit always wins. Our hookless tire guide covers the road limits.

Fine-tuning without fooling yourself

Rene Herse's summary is hard to improve on: "When in doubt, let out some air." (Rene Herse) Write down what you ran, so next time you are not starting from zero.

Try it with your own numbers: tire pressure calculator. For road tires, see road tire pressure for 28, 30 and 32mm. Thinking about wider tires? Check your frame in our gravel tire clearance chart.

What we could not confirm

Spot an error? Email [email protected].

Sources