A vehicle may have a seat and front wheel similar to those of a bicycle. However, the vehicle may have two rear wheels. The position of the two rear wheels may be adjustable. When the two rear wheels touch each other, the vehicle may function as a bicycle. When they are moved apart, the vehicle may function as a tricycle. In tricycle mode, the vehicle may travel autonomously without a human rider. In bicycle mode, a human may ride the vehicle like a bicycle. To transition between bicycle and tricycle modes, each rear wheel may rotate about a horizontal axis that is parallel to a longitudinal axis of the vehicle. In some cases, a bike chain is connected to only one of the rear wheels. In bicycle mode, motion imparted by the chain to one rear wheel may be transferred to the other by friction.