You should inflate them to the pressure recommended by the vehicle manufacturer for your year make and model. This information is in your owners manual or on a sticker on the drivers door frame.
Make, model and year of vehicle would be a big help.
Most cars I've driven have needed about 30psi, plus or minus 4psi, but check the user manual or any stickers there may be.
The 50 psi is the MAXIMUM rating, so don't go over 40psi cold - I've had an over-inflated tyre (the pressure gauge was faulty on the air-line) explode because it was on a car parked in full sunlight on a hot day. It sounded like a bomb going off and it was lucky no-one was near it. Did my car no good, either.
Start with the numbers recommended for your car: they are on a sticker inside the driver's door frame, or in the owner's manual.
DO NOT inflate your tires to the pressure embossed on the tire sidewall. Your car won't be safe to drive.
That sticker on the door jab is for the original tires that came with the vehicle. If at any point the rims or tires have been changed go with what printed on the tire. if you stick with stock rims and the manufacture recommended tire size that number shouldnt change that much. If you over inflate tires the middle will wear out first and if you under inflate the sides will wear out faster. Now onto science when a tire rotates it creates friction and the main result of friction is heat so if you fill the tires to 50psi and drive a lot of freeway over the course of that drive the tire psi will rise not much but it will go up. The old school way of thinking is slightly over inflate your tires to reduce the surface area of the tire well that causes the middle of the tire to wear quicker. the odd tire out here is the low profile tires they NEED that pressure to run without damaging the rim. If it was me and on all my cars I put the pressure at a few pounds below the max. unless I had a truck that was towing or low profile tires then I would get close to that max.
Whatever is listed on the door jamb or in your book, +2 lbs.
