A sucker rod string is formed from sucker rods and sucker rod couplings. The sucker rod couplings are formed from a spinodally-hardened copper alloy comprising from about 8 to about 20 wt % nickel, and from about 5 to about 11 wt % tin, the remaining balance being copper, and having a sliding coefficient of friction of 0.4 or less when measured against carbon steel. The sucker rod string has low friction and improved pumping stroke, enhanced pumping capacity, and less load in the overall system.