It’s impossible to give you an accurate guide for tyre pressures in terms of just PSI. The right tyre pressure is determined by so many factors, some of which include tyre size, speed you are driving, terrain you are on and weight applied to each tyre.
We recently did the Gibb River Road, and my brother towed our off road boat trailer down. As much as we’d have loved to go to Kalumbaru, we just didn’t have enough time, and the boat trailer came purely because we didn’t want to leave it in Broome.
We run Goodyear Wrangler tyres on the trailer, in 285/65/16, or the equivalent of a 33 inch tyre. The trailer, boat and gear inside weighs under 450kg, and as a result the tyre pressures we run often vary a lot from what people would expect.
On the road, anything from 25 to 35 PSI is more than enough. On soft sand, I’ll drop it to 6PSI with no worries in the world.
On the Gibb River Road, we ran the tyres at 9 PSI, and they handled it just fine, doing up to 100km/h in sections. The reason you can go so low with the tyre pressures is that they are such big tyres, and there’s so little weight on top of them.
At only about 225kg of weight on each tyre, with tyres much larger than they could be there’s minimal deflection in the tyres sidewall, which means little heat. As they don’t do any driving or turning there’s almost no chance of the tyre coming off the rim, which means you can afford to let them right down.
Why such low tyre pressures?
In terms of looking after everything, the lower the tyre pressures the better.
The lower they are, the more bumps they soak up, leaving less work for the suspension to do. Aluminium boat hulls can crack easily if they aren’t well supported, and reducing the shock that the hull takes can easily be done by having low tyre pressures. It also means everything in the boat, and the outboard takes less strain too.