(See truck in my sig)...I'd say go with the 3:73 if towing in the mountains...I have the 4:10 due to that was what I was use to. I do 99% of my towing in the mountains of NC, TN, VA and WVa. Even several areas of interstated will be 7%, but some secondary roads I tow on can be much steeper, very curvy and quite mountainous...thus the 4:10 works great, but believe the 3:73 would be the"best of both worlds"....hate to admit it, but when I ordered my truck back in Dec of 12, I didn't know you could order it with a 3:73, (yeah, stupid me), anyway. I tow between 55-65 mph, though I've recently reached speeds of 70 mph while towing and backed it down when I realized it. The truck tows my 16,000# 5er fairly effortlessly.....With the Aisin, got all the HP and TQ I need and having just run my truck and 5er across the scales am well under my trucks GVWR, (WAAYY under it's RAWR), and GCWR with 4:10 is 37,600#, and I"m at 25,240#..(lot's of room for growth:)).. I got 11.3 mpg hand calc. towing approx. 100 miles this trip...