A three-wheel vehicle has two forward steered wheels and one rear powered wheel operatively connected to an engine disposed on a frame assembly. A straddle-type seat is disposed between the forward and rear wheels. A rear swing arm is pivotally connected at a first end to the frame at a pivot point and rotatably supports the rear wheel at a second end. A concentric sprocket assembly having first and second sprockets is attached to the swing arm at the pivot point. A transmission member operatively connects an output shaft of the engine and the first sprocket and a second endless flexible transmission member is operatively connnected between the rear wheel and the second sprocket. An eccentric endless flexible transmission member tension adjustment mechanism is attached to the swing arm to move the concentric assembly sprocket relative to a longitudinal axis of the swing arm.