Taller tires are heavier which means it take more power to turn them and thus more fuel. And you'll be downshifting more often. The experience on here is that larger/heavier tires = worse fuel economy, not better. Similarly, for those with larger tires, going to shorter gearing improves fuel economy.

Regarding what Gene listed, this is true, so acceleration will be great. However, maintaining top speed in overdrive becomes a problem like above, not enough power. Not that there is much difference between 3.55 and 3.73, but 3.08 might be too much to hold 6th gear in a 4WD Screw on anything but Kansas flat roads.

 

APT, while you make a good point about larger/heavier tires, my experience hasn't been bad in the past. 2004 Explorer...stock was P235/70R16 and I upgraded to LT265/75R16 (9% larger tires and heavier by who knows how much) and I only lost 2% MPG (.357 MPG) with all milage corrected for tire size (I think is was less then that, but I don't have more recent data with me currently). That was with a 4.0L V6 and 3.55. As I said..."in the distant future"...just trying to share past experience as I track MPG and how everything I do/change effects it.

 

I concur with all said here. Go 3.73. I'll also say that if you are going with the actory 20's, you'd better get the lowest (3.73) gear ratio available. I can give you several examples where the simple tire size screwed up my towing or accelleration. I ordered 20's on a 2008 King Ranch. The heavy Michelin LT/AT2's made the truck a dog off the line, and moving a 4,500 pound trailer a slower process as well. Now, my 2010 (although it has a lower 1st gear than the 2008) has 18" wheels and easily pulls the trailer, but still downshifts a bit. Even my Dad's identically-equipped 250's to mine in the past (mine with 20's, his with 18's) were always noticeably quicker off the line.