The short answer is you should fill road bicycle tires to 120psi and mountain bike tires to 50psi but keep reading for why and how to tell what your bike is really built for. Most bicycle tires actually have the psi rating written on the sidewall, so unless your tires are really old or dirty you should just look at the side of the tire for a rating. This will tell you how full to fill your tubes. Additionally, you will need a pump with a pressure gauge such as the one below.
I own a mountain bike and a road bike and they each have different pressure ratings. This is because mountain bikes need more grip and have a much larger tube. Road bikes by contrast have smaller tubes that are designed to run at high pressure for a smoother more efficient ride. Road bike tires are also more susceptible to pinch flats if they are not fully inflated. This happens when the tube is pinched between the tire and rim, to avoid this you should check your pressure every few days or even more often depending on how frequently you ride.
- for road bikes check the sidewall rating on your tire and consider the range 115psi to 125psi
- usually your rear tire should be a higher psi than your front because more weight goes onto it
- consider your own weight and the riding conditions, less full equals more grip, but not full enough equals pinch flats
- since I am light, weighing in at 140lbs, I run my road bike tubes at only 105psi in the front and 110psi in the rear
- for mountain bikes check the sidewall rating of your tires and consider the range 40psi to 60psi
- usually your rear tire should be a higher psi than your front because more weight goes onto it
- consider your own weight and the riding conditions, less full equals more grip, but not full enough equals pinch flats
- when riding in sand or muddy conditions, run your tubes at a lower pressure for more grip
- if you are jumping or going over rocks and feel your rim hit the ground through your tube and bottom out, consider adding more pressure
- if you are riding on the street or other hard surfaces, run your tubes at a higher pressure for increased efficiency
It’s worth noting here that according to several of my friends who work at bike shops, it is unlikely that you will cause damage to your bicycle tires or rims by over inflating a tube. This includes the compressed one shot tire inflation devices that many people stow in their portable kits. Just remember that if your tire is too full your ride will become skittish and you are more likely to slide out or have trouble climbing and braking.