Use your tire gauge ONLY and try letting them down 1/2 lb at a time until you get your best 60 ft. times don't forget that all tire gauges DO NOT read exactly the same I have three different low pressure tire gauges, 2 Moroso, one high dollar C gauge, you guessed it, they do not read the same on the same tire on the same day one after the other. I keep one in the car and use ONLY it on my tires I don't loan it out, EVER I learned that lesson the hard way twice, loaned two different ones out and they both came back reading 5 lbs or more with no pressure in them, NO MASS It seems to me the last set of M/T drag tires like your using (I use the 13.5x30x15 ET Streets and the 315x60x15 ET Street Radials and the M/T 10.5x31x15W drag slicks on my car now) my car liked below 10 lbs on my gauges for the best 60 ft. times