twin axles and only 500kg. I would go for low 20's as its not much weight per tyre. My mate has a bike engined westfield that weighs a little over 600KG's with a couple of people and fuel. Its on 13's and he only puts about 22PSI in his as he wore a set of tyres funny due to over inlfating them to the normal sort car pressures, 28/30PSI.