Source: Federal Reserve Bank of St. Louis  

Release: Interest Rate Spreads

Units:  Percent, Not Seasonally Adjusted

Frequency:  Monthly

Series is calculated as the spread between 3-Month Treasury Bill: Secondary Market Rate (ROUND_B1_CLOSE_13WK_2M)) and Effective Federal Funds Rate (https://fred.stlouisfed.org/series/EFFRM).
Starting with the update on June 21, 2019, the Treasury bond data used in calculating interest rate spreads is obtained directly from the U.S. Treasury Department.