I am a Newbie with a 36 spoke 20" Schwinn.
I weigh 170 lbs.
How much air pressure should I be using?
Tire is a 20" by 1.75" street type tread.
Tire states 40 psi maximum.
From my vantage point, it still looks kind of low as I ride…
.(yea, I know “looks don’t count for much”)
Harry 
You should be able to safely put 60 psig in the tire. Tubes are cheap at about 5 bucks if it blows. I run my 35 psig rated Coker tires at 50 psig all the time. I run 20" freestyle tires at 60 psig. I weigh about 200 lbs but that really has no bearing on the safe pressure, just the tire footprint.
Just like the steel rims on my Coker and 20".
Try it at 60, and if the tire feels too hard lower it. You probably won’t have a problem at 60, but people’s preferences vary.
