I'm running 215/35/18s and I have them up at 38 - 40psi depending on the weather. In warmer weather I take them down to 38 or less because the heat in the road will inevitably increase the pressure in the tyres due to thermal expansion. Yeh, the ride might be a little more bumpy than normal but it will depend on the roads you drive on and will benefit your fuel efficiency by a little. I reckon the minimum you run on would be 35, but it all depends on what you like. You don't want to hit a pothole having under inflated tyres and cracking one of your rims, being the worst case scenario and costing you more in the long run.