Is the air pressure the same in bicycle tires and car tires and why would it be different?

It is not the same, as car tires contain much greater volumes of air. The same amount of air that would raise the pressure slightly on a car tire could over-inflate a bike tire. Motorcycle tires typically fall in the middle of these tire sizes.

A car tire contains a greater VOLUME of air at a lower PRESSURE. A bicycle tire has a lesser VOLUME of air at a GREATER PRESSURE. Some bicycle tires can be inflated to well over 90 PSI. The tires on my touring bike are inflated to 75 PSI. That's WELL over what should be in most passenger car tires. Some things to remember: When looking for the correct tire pressure for an automobile DO NOT go by what's printed on the tire, that is the MAXIMUM PRESSURE FOR MAXIMUM LOAD!

YOU CAN CAUSE A BLOW-OUT BY INFLATING YOUR CAR TIRES INCORRECTLY!

There is usually a sticker on the driver's side door jamb that tells what the pressure should be.

OR CHECK THE OWNERS MANUAL

Be sure to CHECK THE PRESSURE WHEN THE TIRE IS COLD! Oddly enough the CORRECT pressure, or range of pressures, is printed on the side wall of most all bike tires. It's important to check for the correct tire pressure on your bike to avoid "pinch" flats and to protect the rims and tires. It's normal for both car and bike tires to lose pressure if they are left standing for any time.