My general take is that bigger tyres require lower presure to give the same contact area. For 195's I would say 20 - 21 is about right. I used to run mt 215's at 19 -20psi and my currect 31's are running at 17psi with perfect road manners. There is a complicated formula to work out pressure but I don't have access to it here. Will try and find it later.