Home/PostalMag/Postal TSP/TSP Rates of Return August 2018 Postal TSP TSP Rates of Return August 2018 PostalMag · Friday, September 07, 2018 Except for the I Fund, the Thrift Savings Plan had a strong month for August 2018. The C and S Funds had particularly strong returns, at 3.26% and 4.57%. Overall, all TSP funds except the F Fund are in positive territory for the last 12 months.