I have 3.55's paired up with my 5.4. I am running LT265/70/17's and have an Edge Evolution programmer. When towing heavy, (5K+) the programmer made a significant difference starting from a stop. (and on steep mountains) At highway speed, the difference wasn't real noticable especially on rolling hills. A smaller tire size will help more when on the highway going over hills.

For my truck LT245/70/17's are recommended for heavy towing. I pull a travel trailer weighing 7500 lbs when loaded, but don't have any issues pulling that much.

Remember, your truck is not a diesel and it isn't a race out there.

 

The stock tire size on the 4x4s was 265/70/17 was it not?

I am really looking for improvements with starting from a stop with a load and going up steep grades. It seems to do fine on the highway although the MPGs are averaging around 9 or 10 at best.