I use 3.08 gears. I have no problem towing 10,000 lb trailers, sometimes through hilly country. These things have staggering torque as low as 800 RPM.
If he is a 4x4, the numerically lowest gears he can get for his front axle is 3.55 and the improvement is not worth the trouble. For 4x4s an overdrive works better. It would give you an overall 2.98:1 ratio. Overdrives are pricey, though. I could not make one make sense and I drive 25,000 miles a year.
A ratio lower than 3.55 could be trouble for an automatic transmission.
Ford now offers a 3.55 ratio as standard on diesel pickups.
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2000 Ford F-350 SC 4x2 6 Speed Manual
4" Slam
3.08:1 gears and Gear Vendor Overdrive
Rubber Conveyor Belt Air Dam
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