The pumping system used in a step ratio automatic transmission can consume up to 20% of the total power required to operate a typical automotive transmission through the EPA city cycle. As such, it represents an area manufacturers have focused their efforts towards in their quest to obtain improved transmission efficiency. This paper will discuss the history of automatic transmission pumps that develop up to 300 psi along with a description of the factors used to size pumps and establish pump flow requirements. The various types of pumps used in current automatic transmissions will be described with a discussion of their characteristics including a comparison based upon observations of their performance. Specific attention will be focused on comparing the volumetric efficiency, mechanical efficiency, overall efficiency, pumping torque and discharge flow.This paper represents information on a wide cross section of current production automatic transmission pumps produced by manufacturers from around the world. The results in this paper are based upon real world applications of these pumps and the various compromises which manufacturers make in order that a pump operate across a broad range of operating conditions as required by the transmission. The conclusions expressed in this paper are based upon a total population of 43 automatic transmission pumps tested at SwRI over a 5 year period. From this population a group of 14 pumps used in 3.0-4.0L engine applications was selected. The conclusions drawn about each pump type are based upon on an average of at least 3 to 4 representative pumps of each type. These pumps included a cross section of pumps of various quality produced by various manufacturers. The data used to support the conclusions in this report incorporate the entire pumping system and the type of packaging and performance constraints placed upon pump manufacturers producing cost-effective, high production pumping systems.