WASHINGTON, May 21 (Reuters) - The Treasury Department said on Thursday it has injected $7.5 billion into GMAC to help it lend to Chrysler car buyers and to partly meet its capital needs.

"This new arrangement with GMAC will help provide a reliable source of financing to both auto dealers and customers seeking to buy new cars," Treasury Secretary Timothy Geithner said in a statement.

The investment is in the form of preferred equity shares so Treasury will not immediately have a common-stock stake in GMAC. (Reporting by Glenn Somerville)