4x4 Tyre Pressures

A tyre's load table gives the load it may carry at each pressure. Find the load on one tyre (half the axle), then the lowest pressure that carries it: that's the least the tyre needs. South African law (reg 238) doesn't allow a wheel on a public road, gravel roads included, to carry more than SANS 1550 allows for its tyre, judged at the cold pressure. This page assumes SANS 1550 matches the tables used here; that hasn't been checked yet.

The tables stop at a lowest pressure: 1.5 bar for passenger-type tyres, 2.4 bar for LT tyres. Below that, nobody publishes what the tyre can carry, so this page shows no floor there. It only shows what tyre makers say about speed.

A tyre flattens until the pressure over its footprint balances the air pressure inside it. So the footprint is roughly load ÷ pressure: drop the pressure by a third and the footprint grows by about half. It grows mostly in length, which is what floats you over soft sand.

The lower the pressure, the more the tyre squashes and the more its sidewall flexes on every turn of the wheel. Flexing makes heat, and heat is what destroys tyres. That's why every maker that allows low pressures ties them to low speeds.

Military trucks with central tyre inflation make the link explicit: each lower pressure setting comes with a lower top speed. These are big truck tyres, so the numbers aren't for your 4x4, but the pattern is the same one BFGoodrich gives for 4x4 tyres: below 1.5 bar, 20 km/h or less.

The tyre's bead sits on the rim's bead seat, held by the flange on the outside and, on a hump rim, a small hump on the inside. ETRTO recommends hump rims for tubeless radial tyres up to load index 121, which covers most 4x4 tyres. At low pressure, a hard sideways push (a side slope, a rut, a sharp turn) can roll the bead over the hump and into the well, and the tyre loses its air. BFGoodrich says to run road pressure across steep side slopes. Beadlocks clamp the bead, but they don't change what the air carries, and Ford doesn't approve them on the road.

Set and check pressures cold: with the tyres at the temperature of the air around them, before driving. First thing in the morning is easiest; otherwise let the vehicle stand (ETRTO says at least an hour, Toyo and Ford say three).

Driving warms the air inside. A reading 20% or more above the cold figure is normal, so never let air out of warm tyres to get back to the cold number. The law also judges a tyre at its cold pressure.

The weather moves it too: about 1 psi (0.07 bar) for every 5.5°C, so a cold desert morning and a hot afternoon can differ by a few tenths of a bar with the same air inside.

Airing down at the start of the sand? The tyres are usually warm by then, so they'll lose a little more as they cool: check again later. The same goes for pumping up warm tyres: they'll read lower once cold.

Keep tyres away from anything hotter than 90°C, such as brakes or exhausts: ETRTO says that can damage them for good.

No tyre or vehicle maker found gives a corrugation pressure, so this page doesn't either. Research on log trucks found that lower tyre pressures reduce how corrugations form on gravel roads and improve the ride, but those were truck tyres within their makers' load and speed tables, not a figure for a 4x4.

The air you need is the tyre's air volume × the pressure you're adding ÷ the air pressure around you. Inland there's less air pressure, so the same tyre needs a little more pumping. Compressors are often rated at "free flow" with nothing to push against; into a tyre at 2–3 bar they deliver less, so real times are longer.