Does anyone know what the recommended tire pressure is for
33 x 5 (23 x 5) 6 ply tire? They are on a 23-4-35.

According to a 1916 vintage Buick Reference book, These early tires should be filled 20 pounds for each inch of rim width. A 4 inch tire should carry 80 pounds of pressure and a a 4 1/2 inch tire would take 90 pounds. 5 inch would equal 100 pounds. I personally would not put more than 80 to 90 pounds in them myself. That kind of pressure is nothing to be playing with. Be sure everything is mounted correctly and the rims and tires are in good shape. Take them somewhere that has a cage to be safe.

We had a local fellow get killed here a number of years ago when the sidewall came apart in a tire and hit him in the chest. Dave!

After 12 years of Touring, I have found 55 psi to be best for my 33 x 4 1/2 (original style high pressure) front tires on my 1923 45.

I run balloon style tires in the rear (24 x 5.00) and run these at 35. The balloons in the rear give about twice the 'footprint' contact patch on the ground which is very important for stopping when all you have is rear brakes. They also last about 3 times as long as the 33 x 4.5s I had back there once.

I can't seem to wear the 33 x 4.5's on the front, they are still in good shape after 12 years.