And if you're running tubeless, you can use even less! If you're accustomed to riding narrow tires with tubes and haven't altered your tire pressure since switching to 29- or 27.5-inch wheels and today's wider tires set up tubeless, you could very well be pumping them up too high out of habit. Hard tires don't conform around trail features so grip is compromised, and they don't help absorb bumps in the trail, making for a rougher ride. Not enough air will cause pinch flats if you are running tubes or the tire to feel squirmy, and in the worst case, cause the tire to roll, which can lead to a loss of control. Too low of pressure can also cause tire and rim damage.
How much pressure you need depends on a variety of factors, including rider size/weight, riding style, rim/tire combo, tire width and whether or not you have tubes installed, which requires a higher pressure to avoid pinch flats. Wheel size is also a factor, as is tire casing thickness. Tires with thinner casing may require a bit more pressure, and downhill tires with thicker casing often need less.
To determine a starting point for many modern trail bike setups, we like this formula we learned from Stan's awhile back:
Rider weight (in lbs) / 7 = x
x -1 = Front Pressure (in psi)
x + 2 = Rear Pressure (in psi)
This is a quick way to find a ballpark number that may be good for many riders. Once you determine PSI with this formula, go for a ride on a trail you know. Pay attention to things like traction and grip around corners and whether or not you are 'ping ponging' off of rocks. If you feel you need to tweak the pressure, experiment by riding a few more laps and letting out or adding a little bit of pressure at a time. Be careful when letting air out, and if you notice excess squish and squirm or tendency to burp the tire in the case of tubeless, pinch flatting with tubes (which means your pressure is too low), or rim hits, bring the pressure back up a little.
Proper pressure numbers will be different for every rider, and the more you start noticing how more or less pressure affects ride quality and tire performance, the more you'll be able to dial in your pressure for a variety of conditions and tire sizes.
A digital tire gauge can be a handy way to check your exact pressure on the fly, and remember to check your pressure and top off your tires with a floor pump before every ride. The next time you're in the shop, ask a Path tech for additional tire pressure tips to learn more!