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

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)

Starting pressures for 40mm tires

Calculator: tubeless, 25mm internal rim, gravel setting, default 42/58 split (updated on 3 October 2026 to match the calculator's per-wheel weight 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.

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:

Above 37mm the figures are extrapolated, not read from Berto's chart; how, and how they compare with Vittoria's, is under Sources and notes.

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 31 / 41 psi on a 21mm rim and 30 / 39 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:

• Gravel: 12.5% below the pavement figure, BikeLab's gravel correction. (BikeLab Studio)
• Rough road: 5% below, BikeLab's rough-asphalt correction.
• Mixed: 9% below. That is our own midpoint, not a published figure.

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

The calculator works out each wheel from its own share of the weight. It assumes 42% of the weight on the front wheel and 58% on the rear for gravel (our default; change it in the tool if your bike differs). Its − and + buttons move the front share by up to 5 points either way, so from 37% to 47%.

Applied through the formula, a static split like that leaves the front noticeably softer than the rear: 28 / 37 psi for 80 kg on 40mm. Some riders like that. Others find the front vague under braking, when it carries far more than its static share of the load. If that is you, add front share with the + button. Vittoria's figures run about 5% apart front to rear, so at 80 kg its front is firmer than ours and its rear softer. A bike with bags on the back needs less front share, and 40/60 is within the calculator's range.

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)

Thinking of lighter tubes? See are TPU inner tubes worth it.

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 caps both wheels at these limits, and at 4.0 bar (58 psi) for 35 to 39mm, and tells you when it does. At a cap it rounds psi down so it never shows a number above the limit: 50 psi for 3.5 bar and 43 psi for 3.0 bar. Your rim or tire maker's lower limit always wins. Our hookless tire guide covers the road limits.

Fine-tuning without fooling yourself

• Check your gauge. BikeRadar quotes SILCA's Josh Poertner: "Your average floor pump has a [pressure] gauge that's +/- 5 per cent accurate." (BikeRadar) At 35 psi, 5% is nearly 2 psi, the size of the changes you are about to make.
• Change one thing at a time, in 1 to 2 psi steps, on a loop you know.
• Too low: the rim hits rocks, the tire squirms in corners, tubeless tires burp.
• Too high: the bike skips over washboard and the rear lets go on loose climbs. BikeRadar reports SILCA's finding that "rolling resistance increases rapidly once you go past the optimum pressure." (BikeRadar)
• Loaded for a trip? Weigh yourself holding the packed bike and use the total. Bags mostly go on the rear, so consider moving the split toward the rear; 40/60 is within the calculator's range.

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.

To keep your pressures by the pump, print the tire pressure card.

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.