Can we helpCar Advice

Reader help: what tyre pressures should I run for my Toyota LC200?

A reader wants help determining the correct tyre pressures for their toyota LandCruiser 200 Series when driving on-road and in sand. 

Dear Practical Motoring,

I’ve been using Robert’s excellent 4WD Handbook to help understand my tyre pressures. In Chapters 4,5,6,7 you suggest pressure reductions using the vehicle’s placarded pressures as the starting point. It is a simple concept that is easily remembered.

However, my 200 series LandCruiser has a placarded pressure of 230kpa (about 34psi). After the first set of tyres wore badly, my Toyota dealer said that 40 – 41 psi was the correct highway pressure; and to alter tyre pressures from that starting point. They said the 34psi was a gravel road pressure, reflecting the more typical use for this vehicle.

My question is: which pressure do you mean your readers to start from?

This checking came about because I need lots more sand driving experience and want to find my own suitable tyre pressure. I measured tyre wall height at 40psi (ie 155mm / 100%), then reduced pressure till it was down to 75% (ie 116mm). Air pressure was then 14psi. Tyre looked OK, contact patch was 75% longer, and it seems like a good pressure for my vehicle, load and tyre.

What do you think of this?

Hello P
A short question with a long answer! Let’s take road driving first.
The placard pressure is definitely just a starting point. The best pressure may be higher or lower. There’s two basic scenarios for 4WD tyre pressures – speed/load and traction. Load is for higher speed work when you consider how much weight the tyre has to carry, and the more weight, the greater the pressure. Same for speed, the higher the speed, the greater the pressure required. Also, “weight”  includes a towball mass, so if you tow heavy trailer increase your pressures on the rear tyres. The front ones don’t need to be increase as there’s actually reduced load on the front with a heavy trailer (more on that here).
Traction is different – here you’re reducing pressure (airing down) which many would say you to do to “increase the contact patch” but that’s not why, it’s an effect. The reason airing down is important is so that the tyre (and car) doesn’t bounce as much, puncture resistance is improved, rolling resistance on soft terrain is reduced, and the tyre wraps around the ground or loose things like stones which aids grip.
When you lower tyre pressures you are speed limited as the tyre deforms a lot as it rolls which rapidly builds up heat and that will quickly damage the carcass, leading to structural failure sooner or later.
So with that background, back to your question, which I’ll take in part as wanting to know the correct highway, road-driving pressures. The placard pressure is for an unloaded vehicle, but often manufacturers give two, three or even four pressures depending on vehicle load.  I don’t recall if that’s the case for the 200, but if yours is modified then it is probably 200-400kg over tare weight and therefore pressures should be increased. You also probably run offroad tyres in light-truck (LT) construction which cannot dissipate heat very well due to their thicker, stronger build, so again a reason to increase pressures.
Now the question of how much pressure to run. There are a couple of rules of thumb. First is to look for an increase of 3-4psi from tyre-cold temperatures after 20 minutes of running, on a moderately temperate day (not a scorcher).  The second is to find a very flat bit of concrete and the tyre widthways with chalk, then drive forwards straight for several metres. The chalk should wear evenly. If it is worn on the outside of the tyre’s width that’s underinflation, on the centre of the tyre is overinflation.
Regardless of your method, regularly check your tyres for uneven wear; angled scuff marks, and uneven wear as per the chalk test above. This indicates either poor inflation or potentially incorrect wheel alignment which is not related to pressure but will increase tyre wear and worsen handling.
As a bottom line, I’d agree with your dealer that 40psi would work well for a modified 4WD for the reasons stated above, but they’re wrong to say that 34psi is for gravel use. 
If there is any doubt about tyre pressure on-road opt for higher rather than lower. Overly high pressures can damage tyres and lead to poor handling, but they are a lesser evil than low pressures which kill tyres and mean even worse, potentially lethal handling.
One tip – if you ever notice the stability control system cutting in when it never did before that can be a sign of overly low tyre pressures.
Now for sand driving.
There’s no easy way to measure the correct sand driving pressures. In my book I give approximate percentages of placard, and that’s a good start, but much depends on vehicle weight – more weight, higher pressure. Even more depends on the nature of the sand. The softer the sand, the lower the pressure.
Most 4WDs like the LC200 can run at 17-18psi quite comfortably, and 14-15 with care. You can go as low as 8, but that’s for extreme situations only. The tradeoff is this; the lower you go, the better the flotation, but the tyre becomes less well fixed to the rim and you run the risk of the tyre seperating from the rim (debeading). You are also severely speed limited at lower pressures. But if we were talking of a Suzuki Jimny which runs low 20s on road then you’d start your sand work at 10psi.
For the LC200, if the sand is quite hard packed, say inland desert or a hard beach like this:
then maybe 25psi or even 30psi is all you need. I once got off a beach driven onto a road pressures of 40 by dropping to 35.
On the other hand, if the sand is like this:
then you may need to drop pressures really low, maybe start at 20psi and expect to go to 15. A good hint is that if you can’t see tyre marks in the sand it must be soft.
I would recommend not constantly driving at the lowest pressures the car can handle because you want to keep a bit in reserve. Get stuck with 14psi and you can drop to 10… get stuck with 10 and there’s not much lower you can go.
Sand driving pressures are all a matter of judgement. One tip though – carry Maxtrax or similar just in case!

Robert Pepper

Robert Pepper