Having the right tire for the discipline the rider will practice is very important. Knobby tires will perform better on downhill and technical terrain, but will roll worse and slower.
Having the right pressure is important. All tires will have the range of pressure suggested on the side of it. The right pressure is based on preference. Trying different settings on both front and back tires is recommended for every rider.
Size and width
Mountain bike tires width usually go from 1.75″ to 2.5″. Where 2.5″ are usually used by downhill bikes. XC bikes will usually go from 2.2″ to 2.35″.
Size depends on the bike. 26″ tires should be used on 26″ bikes. That said, there are some practices like running 29″ tires (and wheels) on 26″ fatbikes. Because they have enough clearance.
A very important item on XC bikes. Finding the right balance between weight and durability is key. Lighter tires will puncture more but roll much faster, heavier tires are usually wider and have better traction and puncture resistance, but they roll slower.
Typical tires will have the following difference in weight:
|Cross Country Tire||Downhill Tire|
Most high performance mountain bikes nowadays are going tubeless. They usually come with tubes in the tires but they are marked as TL-ready. This allows the tire to inflate without a tube, using sealant to avoid the air to leak.
The benefits are many: saving weight, puncture resistance, rolling weight, etc. The process isn’t complicated to do, but it’s recommended to be done by someone with experience as the sealant can be hard to control.