I went from a 3.54 CTD to a 4.10 CTD and what an amazing improvement in towing balls! Before my 3.54 CTD I had a 1/2t 360 ci gasser with 3.92 and it was terrific towing 5000 lbs. Stock tire height , 70 mph was about 3100 rpm in direct and the engine loved it. That truck still got 17 mpg hiway. For your smaller 4.6 I don't think id go lower than 4.10 for towing in direct and save OD for empty. My 4.10 CTD dually gets between 18.3 -19.8 empty depending on city or hiway and AC on or off. 4.10s don't hurt mpg as bad as you might think. Craig