How much air should I pit on my tires?

factory specs at a minimum, and some of us like to run as high as the max sidewall pressure. keep the rear 2 psi below the front.

 

It varies based on the brand and model of tire, but if you look at the tiny print on side of the tire it will tell you what the maximum safe pressure for that tire is... and that's the max sidewall pressure.

 

Go read the labels molded into the sidewall of your tires. Maximum labeled pressure varies by tire make and model, and on a five year old car, the tires are likely to be replacements, not the original tires from the factory. 44 psi is most common, but 51 is also frequent, and I wouldn't rule out the possibility of finding something labeled just 35.

'Best' tire pressure is a compromise among several competing factors. You will have to make your own choice based on your own weighting of these factors. Without good personal reasons to the contrary, stay within the range mentioned by Bisco above.

 

Your Prius has been tested and engineered to specific specifications. The correct tire pressure is located on the door jam. The number given was arrived at by extensive tests by toyota and the tire manufacturers. Should you wish to increase the pressure to obtain better gas milage as many owners choose to do, you will sacrafice many of the characterisitics built into the tire such as good braking, handling, ride, and tire wear. I use the numbers on the door and achieve fantastic milage (53 - 56 MPG) and ride comfortably and as safely as the Prius was designed for. I admit trying a higher tire pressure for better fuel economy but I was unwilling to sacrafice the pleasant ride and safety for better numbers.

 

I own stock in a tire company. I thank every American who likes to run their tires at low pressure causing premature wear.

 

I used to think that too. Before my Honda hydroplaned much too easily at its comparatively low placard pressure. And before a too-low placard pressure was implicated as one of several factors that lead the Firestone / Ford Explorer rollover fiasco and its many fatalities.

Now I know better.