Not all tire gauges read the exact same pressures, start high and look at the tire wear pattern and the time slip while going down 1/4 to 1/2 lbs at a time on the pressures to get the best results in your 60 ft times Are you running tubes and what width rims are those tires on? What RPM do you leave at?
On your deal I would start at 14 or 15 lbs and go from there If it dead hooks at that pressure go up 1 lb. at a time until it spins and then come back down to the best 60 ft. times
The last pair of 9x30x15 bias ply slicks I ran on my old M.W. stocker over 20 yrs ago had 18 lbs in them, they where mounted on 10 inch wide wheels and we where eleaving from 2700 to 3500 RPM foot braking, 1.42 60 ft times That was long before the drag radial tires where introduced