The circumference of a circle is πd or pi multiplied by the diameter of the circle

To determine how far your robot will go at 45.5 RPM, you need to know the rollout of your wheel. 

This is a fancy term that simply means the distance a single rotation of the wheel will take you. It's easy to calculate your rollout. 

Remember that the circumference of a circle is πd or pi multiplied by the diameter of the circle. 

 
So if we use a 3-inch (7.62-centimeter) wheel, we will travel 9.4 inches (24 centimeters) per revolution (which takes 1.32 seconds) or 7.1 inches per second (18.1 centimeters), which  is 35.7 feet (10.9 meters) per minute, which is about 0.41 miles (0.65 kilometers) per hour. 

Not very fast it seems, but this is a comfortable speed for a small indoor robot.