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Since the 4.6 likes to rev so high and develops it's power at higher rpms, I would go with at least 4.10's, and lean to 4.30's if you offroad with it or live in an area with alot of hills. I run 4.56's with my 33's because I haul a camper and pull a trailer and the 351 in my F350 needed some gearing help to get the job done. Since you likely have overdrive, there isn't much reason not to have the lower (numerically higher) ratio gears. The overdrive gear will take care of freeway cruising rpms so still get good mileage at reasonable highway speeds.