General Tyre pressure calc
Aidan: There's a rule of thumb method for standard tyres.
There's a method for calculating for other tyres that have got the load figures stamped on them, ie max pressure 65psi at 850kg sort of thing.
You take half the axle rating and ratio the psi against the load, so front axle 1300kg, so 650 is half, 650/850 x 65 = 49psi.
I found if you worked this out on a standard tyre you got pretty much the same figure as in the handbook and use this as a guide to working out pressure for 15" tyres and then trial and experience do they drive nice, fuel economy ok, wear even, ended up 3 bar rear 2.5bar front (which is the same as 205R14Cs)for ColwayMTs (they are a retread remember )and they've been fine at that, air down to 25psi when required for grip/footprint got 4 years out of them and there's still a bit of life left for the project in them.