I think you are great candidate for 4.88s. I had the same annoying tranny kick-down from overdrive to direct on the slightest grades with the stock 3.73s and (tall) 255/85R16 tires. With my 4.88s I can usually maintain freeway speeds without loosing overdrive unless I have cruise control on (which kicks-down too easy). Although cruise control is still too quick to down shift the tranny, I find I can use cruise much more than I did before because the propensity to downshift on grades is greatly reduced. On one long freeway grade approaching my house, the car actually prefers to be spinning a bit faster, about 70, to easily keep the tranny locked-up in 5th. I’m not sure the engine would be spinning fast enough at 65-70 with 4.56s?
I think you might be satisfied with 4.56s, but since you are thinking about bigger tires in the future I would really lean toward 4.88s. Although I had some hesitation at first, when it comes to mountain driving, and particularly towing, lower gears are just generally much better. Since gearing essentially multiplies torque, we have to ask ourselves, wouldn't we like a little more torque?
I plan to post some additional details in a future post, but because my F350 was broken this past autumn I towed my big, heavy travel trailer to a couple hunting camps (30-ft & 7,500-lb). Of course I do have the little V8 in my 4Runner and it is rated to tow 7,000-lbs but I can't imagine being happy pulling that much weight with the stock 3.73s! Let me just say that my hunting partner who was pulling a smaller/lighter trailer with his 2006 Dodge/Cummins was impressed.
Either way, gearing down is very good
