Last updated 6/12/2015
I’ve found there’s a lack of clarity when it comes to information on tyre pressure. There’s lots of conflicting information. I’ve read a lot of it and I’ve accumulated a reasonable amount of experience so this article is the culmination of that knowledge. Hopefully it helps you set correct tyre pressures.
The rules for tyre pressure vary according to tyre specifications (max load, max pressure, construction, dimensions etc), type of vehicle, vehicle loading, suspension characteristics and application. There is no single rule that can be universally applied. The information here is just my opinion. It’s my best guess at a rule of thumb that can be applied under any circumstances. It’s not perfect. I am not an expert on tyres. This is my interpretation of the information I’ve read and the experiences I’ve had. First I go through general principles and then apply them to a few circumstances.
As with everything in life, tyre pressure involves compromises. There’s no perfect pressure for any circumstance. Changing pressure to improve something invariably makes something else worse. So a judgement needs to be made. My judgements aren’t necessarily right. However from this article you will know how pressure effects various characteristics and you can modify pressure to achieve what you desire whilst at the same time knowing what other characteristics you are compromising.
Lower pressure improves comfort. Low pressure allows a tyre to deform quickly to any bumps in the road, insulating the vehicle from the impact. Higher pressure reduces tyre flex and makes for a firmer, bumpier ride.
On a hard, smooth surface, pressure does not have much effect on traction, although over-inflated tyres will reduce traction slightly. When encountering bumps, lower pressure provides improved traction. Lower pressure means the tyre stays in better contact with the road as it conforms to bumps. Higher pressure yields less traction over bumps as a tyre bounces and load is temporarily reduced or completely removed if the tyre becomes airborne. A tyre in the air provides no traction at all.
On a surface with hard edges, soft tyres will deform around those edges and provide some mechanical “keying” between the tyre tread and the surface edge. This improves traction. A hard tyre will key less and is more likely to slip across a sharp edge.
Soft tyres are hard to steer. So lower tyre pressure increases the effort required to steer the vehicle. Higher pressure makes it easier to steer.
Sidewalls are the most vulnerable part of a tyre and are damaged by impacting, grinding and slicing across the side of obstacles protruding from the road or track. High tyre pressure reduces sidewall bulge and thus reduces the risk of sidewall damage. Lower pressure increases sidewall bulge and so increases the area of sidewall exposed to potential damage. Low pressure also increases the risk of the tyre tread coming into contact with the rim under an impact load which will damage the tyre. So again lower pressure increases the risk of tyre damage in this case. However higher pressure is not universally better in terms of tyre damage risk. Very hard tyres undergo greater impact loading when hitting an obstacle. Softer tyres will flex. Harder tyres will not flex and thus yield a greater force applied to the contact area, potentially damaging the tread by slicing, chipping off small pieces of rubber, ripping off large chunks of rubber or cutting through the tyre via impact fractures. So lower pressure is better to resist this type of damage to the tread surface. Obviously you can’t have hard and soft tyres at the same time, so a compromise is made. Very soft tyres and very hard tyres are both bad for tyre damage. Something in between is best. Driving slowly and picking good lines will help reduce the risk further.
Some people believe keeping tyres rock hard minimizes tyre damage as the sidewall is the most vulnerable part of the tyre. I’m certain rock hard tyres are more likely to suffer chipping, gouging, cutting and impact fractures. The tread area becomes very vulnerable at high pressures. Hard tyres will also be wearing out suspension faster and increasing the chances of suspension failure. From my observations a common denominator in many vehicles experiencing multiple tyre problems and / or suspension problems is super hard tyres. Of course some people with rock hard tyres will argue that they have never had a problem. Differences in load, driving style and speed may explain this but in general reduced pressure can give you overall reduced risk of tyre and suspension problems.
Another form of tyre damage occurs through overheating (see below under Temperature). Higher pressure reduces the risk of temperature related damage. When running lower pressure, reduce speed so that heat loading does not escalate to dangerous levels.
Lower pressure increases the risk of busting the tyre bead. Higher pressure holds the tyre bead firmly to the rim and thus reduces the risk of busting the bead.
Generally, higher pressure reduces tyre wear, especially at high speed. Soft tyres have more rolling resistance – this is a form of friction and the energy dissipated wears out the tyre. Another factor may be softening of tread compound due to increased temperature associated with lower pressures. So lower pressure increases tyre wear. Very high pressures can increase wear in the centre of the tyre. If you count non-catastrophic chipping and gouging as wear then high pressure also increases this type of wear.
Pneumatic tyres are able to respond quickly to bumps because they have very little inertia to resist movement. Sudden impact loads are easily absorbed by a pneumatic tyre. It’s the reason why pneumatic tyres are so universal. Suspension on the other hand is heavy. To move suspension quickly in response to a sudden impact requires a very large acceleration force to overcome the combined mass of the suspension, tyres, wheels, axles, etc. A hard tyre will transfer more of the load to the suspension, resulting in a severe spike in stress transfer to suspension components as impact loads are encountered. So lower tyre pressure reduces suspension wear. Low tyre pressure increases the work done by a tyres in response to bumps and reduces the force transferred to the suspension. Tyres at higher pressure are not able to react to bumps as well and transfer more force to suspension components.
If you run rock hard tyres many thousands kms on corrugated roads you’ll soon need new suspension bushes. Keep doing it and suspension failure and even chassis failure is on the cards. Millions upon millions of load cycles soon accumulates to significant fatigue and it’s only a matter of time until something breaks. With reduced tyre pressure you can stretch that failure rate out significantly.
At a given speed, a tyre’s ability to carry load is directly related to its pressure. Higher pressure means ability to carry more load. Lower pressure means reduced load carrying ability. Tyre manufacturers produce tables and charts that specify a tyre’s load carrying capacity according to different inflation pressures.
Significant heat is generated by sidewall flex and rolling friction. This means tyres at lower pressure run hotter. Tyres at higher pressure run cooler. Overheating a tyre by under inflation is the most popular way of destroying tyres. Usually it will manifest as a sudden blowout or delamination at speed.
At high speed, heavy loads and high ambient temperatures, a properly inflated tyre may experience temperatures up to 95 deg C. Normal operating temperatures will be less than that. Usually an internal air temperature of around 80 deg C is considered to be a safe for normal conditions. A temperature of more than 100 deg C is a problem and above 120 deg C will see a tyre quickly fail.
Heat load on a tyre increases with speed. This means at higher speed, to maintain a safe tyre temperature, higher pressures are required. Lower pressure increases temperature and reduces a tyre’s ability to withstand high speed. In other words, when running reduced pressure, speed must also be reduced to offset the additional heat load and maintain a safe tyre temperature. At any pressure, increasing speed increases tyre wear.
Important note: At highway speeds, you must run highway pressure, regardless of surface conditions. At highway speeds you need highway pressure to maintain a safe tyre temperature. Highway pressure can be determined by following the instructions in section “General Tyre Pressure for On-Road” below.
On hard surfaces, harder tyres have less rolling resistance and thus yield improved fuel efficiency. So higher pressure improves fuel economy. Lower pressure increases rolling resistance which has to be overcome by burning more fuel. See How to Improve Fuel Efficiency for more details.
On soft surfaces like dirt or sand, hard tyres dig in and create extra drag, causing the engine to work harder and more fuel to be consumed. Soft tyres float on top and do not dig into the surface. So reduced pressure offers improved fuel efficiency when traversing soft surfaces.
General Tyre Pressure for On-Road
Tyre pressure should be measured and set cold. Tyre pressure should be set according to the tyre placard on the vehicle and / or the owner’s manual. This is the minimum tyre pressure for the vehicle under normal on-road driving conditions. The maximum pressure is the pressure rating indicated on a tyre’s sidewall. Never exceed a tyre’s maximum pressure rating.
Increase tyre pressure from that specified by the vehicle manufacturer according to the following criteria:
– if travelling at high speed for long periods of time, for example extended highway runs, increase tyre pressure by 4 psi.
– if carrying a significant load in the order of a couple of hundred kilograms, for example some passengers and their luggage, then increase tyre pressure by 4 psi on the axle carrying that load.
– if carrying a very heavy load, say a fully loaded ute, increase tyre pressure by 6 to 10 psi on the axle carrying that load. An increase of 10 psi would be for say a full tonne in the back of a ute.
– if towing a small to medium load, increase tyre pressure of rear wheels by 4 psi.
– if towing a heavy load, increase tyre pressure of rear wheels by 8 psi.
These pressure increments are cumulative. For example, when set up for touring, my Hilux is loaded pretty heavily with about a tonne in the back, so I increase rear tyres by 10 psi. I’m also doing a lot of highway driving so I increase by a further 4 psi. So that means I am inflating my rear tyres an additional 14 psi over that indicated in the owner’s manual. My front tyres I estimate have an extra couple of hundred kilos of load with the bull bar, stuff at the front of the roof racks and stuff in the back seat which would distribute partly to the front, plus some other miscellaneous items under the hood. That calls for an increase of 4 psi. Again for highway speed I increase a further 4 psi so that the front tyres are running 8 psi higher pressure than the pressure indicated in the owner’s manual.
You may have a situation where the cumulative pressure increments exceed the rated pressure of the tyre. For example if the tyre placard indicates 35 psi and you’re carrying close to a tonne and you’re doing highway work and you’re towing a very heavy load, then the pressure increase would be 10 + 4 + 8 = 22 yielding a total pressure of 57 psi. This high pressure is indicative of the extreme stress and heat load placed on tyres under these conditions. You might be exceeding the tyre’s maximum weight capacity. If the pressure exceeds the maximum tyre rating then use the maximum rating. Do not exceed the maximum rating.
The tyre placard gives the minimum pressure for the factory tyres. Different tyres require different pressures depending on their construction. For example stiffer sidewall tyres generate more heat and thus need to be kept at higher pressures. If you are using the same tyre as the tyre provided from factory, or very similar dimensions and construction of tyre, then the tyre placard is a pretty good starting point. But if you change tyre dimensions or construction (for example go from passenger tyre to 4WD tyre or 4WD tyre to light truck tyre) then you will need to adjust the pressure accordingly. You can use the max pressure and load ratings of the tyres to gauge how much you need to shift pressure but it’s hard to say without full pressure / load charts for each type of tyre.
Speed Derating at Reduced Pressure
The section above gives you the pressure you must have to safely run at the tyre’s speed rating. A speed rating of S corresponds to a maximum speed of 180km/h. How fast can you go at reduced pressure? It’s hard to say. The relationship isn’t perfectly linear, varies from tyre to tyre and depends on many factors that are difficult to account for. However from my research I’ve come up with a rule of thumb that gives an idea as to how slow you need to go at reduced pressure. It may not be too accurate but it’s better than nothing. It’s not very accurate for very low pressure and is mainly useful for mid to high pressure settings.
My rule of thumb is: if you reduce pressure by a factor of x then you need to reduce maximum speed by a factor 2x.
I’ll go through an example to explain. Let’s say I’ve gone through the process above and have determined that the tyre pressure for highway driving is 38 psi. The tyres have a speed rating of 180km/h. Max allowable speed vs pressure would then be:
|pressure reduction (%)||pressure (psi)||speed reduction (%)||speed (km/h)|
So the lowest pressure I can run is half highway pressure (in this case 19psi). Below that and my maximum speed is negative. At these sorts of low pressure speed must be kept to a minimum.
The table above loses accuracy for low pressures. I’ve puttered around in the dunes and at the beach all day at pressure much less than 19psi and the tyres stay cool. On the other hand even with rock hard tyres they soon get freakin hot after a few hundred kilometers at highway speed on a hot day. Accumulation of heat is the problem. Short term deviations above the maximum derated speed are ok. If you are offroading at low pressure and constantly slowing down for obstacles and sight seeing then you shouldn’t have a problem with exceeding the derated speed limit. Use the table more as a guide in situations where you are likely to be moving at reasonable speeds for long periods of time – say for long gravel roads.
Tyre Pressure for Sand
Sand requires low pressure. The softer the sand, the lower the pressure. Reducing pressure makes a massive difference in soft sand. Keep reducing pressure until your tyres float across the top. A typical sand pressure may be around half highway pressure for medium sand or 1/3 highway pressure for very soft sand. Note 1/3 of highway pressure is too low according to the tyre derating formula above, but I’ve never had a problem at low speeds. Keep in mind the increased risk of sidewall damage and busting the bead as pressure is reduced. In emergencies pressure can be reduced further. Do not reduce pressure to the point where the rims are contacting the tread and do not go below around 5 psi to ensure the tyre stays seated on the rim. These extreme low pressures are purely for recovering the vehicle and shouldn’t usually be used. See How To Drive On Sand for more information on correct sand tyre pressure and other details about sand driving.
Tyre Pressure for Rock
Reduced tyre pressure is useful for rock as it improves traction by improving flexibility and helping to maintain tyre contact with the ground. It also improves mechanical “keying” between tread and rock edges which further improves traction. Hard tyres will bounce over rocky terrain and in the process unweighten themselves and lose traction, causing wheel spin and dangerous shock loads through the vehicle’s drivetrain. Low pressure helps reduce the magnitude of jarring impacts that are transferred to the suspension and cabin. Reduced pressure also reduces impact loads on the tyres themselves, reducing the risk of damage. However rock is also risky in terms of sidewall damage. Grinding the sidewall against the side of a rock can cause damage. Also a heavy impact on a sharp rock will heavily deform a tyre at reduced pressure, running the risk of pinching the tyre tread between the rim, causing tyre damage. Similarly very hard tyres are at risk of impact damage and chipping. So rock may call for a pressure about 3/4 highway pressure. Reducing pressure further will help improve traction and stability but expose the tyres to greater risk of sidewall and pinching damage.
Tyre Pressure for Corrugations
Tyre pressure should be reduced for corrugations. There are many benefits afforded by reduced pressure. A significant contribution to corrugation formation is suspension articulation. After a bump, the inertia of the articulating suspension causes it to recede more than the bump, creating an unweighted section of track which will become the peak of a corrugation. As the vehicle advances the suspension’s inertia then causes the recoil to overshoot, increasing pressure on the track and forming the trough of a corrugation. Exiting the trough presents as another bump to the next vehicle which triggers the cycle to continue and another corrugation is formed. This process is reinforced with every vehicle that passes, creating new corrugations and increasing the size of existing corrugations. Reduced tyre pressure means the tyre deforms more and the suspension articulates less, so the suspension’s effect on creating corrugations is reduced. So lower pressure reduces the formation of corrugations. This is a good enough reason on its own for reducing tyre pressure on dirt tracks. Corrugations are expensive to maintain, aren’t fun to drive on and are very bad for a vehicle and its cargo, so any way of preventing them is useful.
Reduced pressure also improves traction, as the tyre remains in better contact with the track surface as the vehicle passes over the corrugations. You may have experienced a dangerous loss of traction when encountering very bad corrugations with the feeling of the rear end of the vehicle drifting sideways across the track. Reduced pressure prevents this situation. Lower pressure also reduces wear on suspension components (see section above on suspension wear), reduces the risk of mechanical failure, improves comfort for occupants and reduces the risk of nuts and bolts coming loose. A typical pressure for a corrugated dirt track may be around 3/4 highway pressure. Extreme corrugations may require say half highway pressure but at such low pressures heat loading may become an issue on long drives and speed must be kept to a minimum.
Tyre Pressure for Mud
Pressure when tackling mud is a compromise between flotation according to the same principle as driving on sand, and bite into firmer soil afforded by harder tyres. Reasonable pressure needs to be maintained to ensure tyre safety when banging around through obstacles and ruts. A starting point may be around 3/4 of highway pressure. Watery mud with a hard base may be ok at full highway pressure. Thick, deep mud with no base at all may require pressure closer to that of sand driving.
What do Trucks Do?
I had a chat to an owner of a 4WD tour business in the Kimblery of WA. He runs trucks with passenger cabins fitted to the tray up and down the Gibb River Road showing tourists around. What pressure does he use on his fleet of trucks for this bumpy and corrugated road? About half highway pressure. For the trucks highway pressure is about 90psi and he runs about 45psi. He does it to reduce risk of tyre damage, reduce suspension maintenance and improve ride quality for passengers. If an owner of a fleet of tucks is reducing pressure then that’s pretty good weight to the argument.
I’ve talked to truckies about tyre pressure on road trains. One truck driver told me how some trucks are fitted with systems to deflate and inflate tyres at the push of a button, similar to what is found on Hummers. It’s often used for road trains doing regular offroad / on road commutes, for example transporting minerals from mines to port. They run reduced pressure for dirt tracks and then pump them up for the highway run. If big road trains are reducing pressure then that’s pretty good weight to the argument.