New results for downhole beam or rod pumps from fairly recent test data are in conflict with traditional relationships. Additional testing is in progress. A pump with large plunger/barrel clearance will cause high fluid slippage. A pump with smaller clearance will cause less fluid slippage, but a tighter fit will tend to increase rod buckling at the pump to a greater degree and may lead to advanced pump and rod wear rates. Therefore, it is important to be able to predict pump slippage and also related froces at the downhole pumo. Considerations for pump leakage and example calculations are presented using the older and the new pump slippage relationships. Aderivation to account for the pump velocity effects on slippage is also presented. The effect of pump clearances on possible rod buckling above the pump is also studied. Further additional possible causes of rod buckling are presented, discussed and compared. The results will help the reader to decide on sizing pump clearances to provide leakage for pump lubrication without losing too much on pump efficiency. Several ideas on the sources of rod buckling, such as flow through the travelling valve, and the plunger-barrel interaction are presented and compared. A review of methods to combat rod buckling above the pump is presented.