27.5-inch wheels (also known as 650B) offer the best characteristics of 26- and 29-inch wheels for an outstanding ride.
Efficiency: Increasing the wheel diameter decreases the angle of attack (the angle at which round objects meet square objects). The lower the angle, the easier it is to roll over obstacles and maintain momentum.
Traction: With a larger diameter, the tire's contact patch increases, putting more rubber on the trail. A 27.5-inch (650B) has nearly the same contact patch as a 29er for superb acceleration, braking and cornering.
Weight: The weight of a 27.5 wheel, tire and tube is only 5% more than an identically built 26-inch wheel. Compare this to a 12% increase with a 29-inch wheel. Lighter wheels mean a lighter bike, and better climbing and acceleration.