Tuesday, Jun 11, 2013 at 15:35
Tyre pressures as stated on the tyre placard are a good starting point, but remember they apply to the original tyres. If, like many offroaders or tourers, you swap tyres then the placard pressures become less relevant. In NSW you are only allowed +15mm of diameter without an engineering certificate, but that allows a change from (potentially) a very "road biased" car tyre to a 15mm larger "off-road" light truck tyre, which is a very different beast.
The light truck tyres on my Hilux state (on the tyre) the maximum load carrying capacity with an inflation pressure of (from memory) 70psi. But Toyota, when they specify 28psi are assuming a lightly loaded vehicle with "car" tyres being driven on quality bitumen - not 800kg of fertiliser on dirt roads with A/T light truck tyres!
Then, of course, you alter the pressures to suit the road. Off-road work generally requires a reduction in pressure - if you have a pump to re-inflate them when you return to the bitumen! Travel over sand and corrugations is also often "eased" by marginally lowering the pressures.
Regarding fuel economy, generally speaking the higher the pressure the better the economy, but at the cost of reduced ride quality and grip (less tyre in contact with the road!). As mentioned in a previous post, reducing the compliance of the tyre also transfers more load to the
suspension.
It used to be said that you should adjust your pressure to give a 10% (if I remember correctly) increase from cold to hot. So if you inflate to 30psi when the tyres are cold, after an hours driving they should read 33psi. If they read 36psi, then they needed more air to start with, and if they read 31psi they needed less. This will vary with load, so there is no single "best" pressure, even if you never leave the bitumen...
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