The side walls quote a pressure just below 45 PSI.
Harris Cyclery quotes 55 psi. I found that at 45 PSI, they were very
cushy but very slow. That might be good on gravel or such. 50-55 PSI
they ran much better.They are only slightly slower than Kenda K40s at the same pressure, but much cushier. They have an "all 'rounder" type tread that is not quite slick, but has small knobs. They handle dirt/sand/stones better than Kendas.
The hardest part is getting them to fit. It's a very close fit in the fenders. I did get them to go OK on my 1974 Sports.

