The TSP C-Fund which approximates the S&P 500, has had an average annual 12.29 percent gain between 1988 and 2020; the TSP F-Fund, a broad index representing the U.S. bond market, has had an average annual 6.29 percent from 1988 to 2020; and the G-fund, long term U.S. Treasury notes, has had an average annual of 4.70 percent gain from 1987 to 2020.
To learn more about the TSP Summary of Returns, go to https://www.tsp.gov/investmentfunds/returns/returnSummary.shtml.
The long term rate of investment return used by the Civil Service Retirement Fund Board of Actuaries is 5.75%. We set the default rate for before retirement at 4.95% and rate of return after retirement at 3.0% with a valid range of 0% to 20% for each. You should choose assumptions--or range of assumptions--that are right for you based on your particular circumstances.