the old rule of thumb as you need minimum of 10 psi for every 1,000 rpms. So if you idle at 700 rpm you need 7 psi. If you run 3,000 going down the highway, you need 30 psi. But, those are just what I always heard growing up, thought I'd share. I've had many old motors run for years with 10 psi at red lights and 25 or so psi going down the highway and I know the rpms where higher than 2500 (likely more like 3500).
