If you believe information from the Internet, a Chihuahua is between 13 and 20 km/h fast.
The speed depends, among other things, on the state of health, fitness, and physique of the dog. Top speeds are usually achieved by the slightly larger Chihuahuas with a very good physical condition.
In San Diego (California) there is even an annual Chihuahua race for the little athletes. The winner is whoever covers a distance of 35 feet (approx. 10.5 meters) the fastest.