Tire Pressure for Mountain Bikes
Good tire pressure hinges on a lot of different factors. How you ride, where you ride, when you ride, what the weather is doing -- these can all have a pretty dramatic effect on what you should be doing with your tires. The best advice is always to start off on the hard side and gradually lower pressure until you've found something that feels like it gives you the best combination of grip, precision, speed, and ride quality. Those are all pretty subjective concepts, too, which makes trying to tell you what pressure you should be running just that much trickier.
Luckily, we've got some firsthand experience and access to just enough science to make us sound like we know what we're talking about. The chart below is a good place to start.
One last point to mention.
These numbers are based on typical tubeless tire setups. If you use innertubes, you'll want to bump these pressure recommendations up by about 10%.