The tyre placard on my 110 has pressures varying from 35/35 to 60/65 depending on load and speed. Notably, these figures are the adjusted ones for Australia, and are about 5psi above the UK figures - see below. I usually run 35/35 empty and 40/50 at the maximum load I normally run.
Tyre pressures are always a compromise. A minimum is set by the pressure that will prevent a dangerous pressure rise (and this depends on ambient temperature and speed as well as load), but other factors may require a higher pressure.
From the tyre manufacturer's point of view, generally the higher the better, as even a small temperature rise will shorten tyre life, especially in areas with high ambient temperatures.
From the car manufacturer's point of view, other factors come in. The lower the pressure the better from the comfort point of view, but higher pressures will generally give better handling, and in particular a car's inherent handling characteristics may require an apparently strange difference front to rear. So the tyre placard figures (which have legal force) are the manufacturer's compromise to give a ride that will not upset passengers too much, and handling that will neither upset drivers nor lead to liability claims.
(Pressure affects both the faithfulness with which the tread follows the direction the wheel is pointing despite side loads, and the amount of and shape of the contact between the tread and the road, although this is less affected with radials than with crossplies)
While the minimum set by temperature rise mentioned above will vary quite a lot between different tyres, depending mainly on construction and rubber composition, the ride and handling is less affected by these factors, and even less by changes in tyre size.
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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Originally Posted by PhilipA
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