The power produced by an engine increases with the engine's RPM to a maximum, then falls away. This is known as the point of maximum power. Given a curve describing the overall drag on the vehicle, it is simple to find the speed at which the total drag forces are the same as the maximum power of the engine. This defines the maximum speed the vehicle is able to reach. The rotational speed of the wheels for that given forward speed is simple to calculate, being the tire circumference multiplied by the RPM. As the tire RPM at maximum speed is not the same as the engine RPM at that power, a transmission is used with a gear ratio to convert one to the other.