You could do a number of mods that may promise hp gains but most have never been tested. Most people just weigh how the mod feels on the pedal or smoothness of the motor. I have a 3.0 and debated doing plenty of mods and I did a few. I have a super chips programmer, JBA headers and new third cat and flowmaster super 44, 10mm plug wires, and CAI. All together idk If those are mods that show measurable gains but those are the most common mods for the 3.0. You could also look into under drive pulleys

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2006 Ford Ranger XL 4x4 single cab with a 3.0, 3 inch PA body lift with gap guards, 1.5 inch torsion bar keys and a 0.5 inch crank, 33x12.50 inch BFG AT KO's, 15x10 inch Eagle Alloy Classic II's, dual in dual out single magnaflow muffler, rolled exhaust tips, new superlift shocks, clear corners, Linex bedliner, toolbox w/ whip antenna, K&N cold air intake, and lots more