I didn't lose any fuel economy going from 4.10 to 4.88 because the engine was finally happy. All 37's are not equal. Something smaller like the OZ might be fine with 4.88, but a bigger 37 (actually 37" tall) and wider width like 14.5 really needs the 5.13s. I don't remember reading many posts where guys regretted going deep. And look at the guys with stock 4.10 or 4.56 1-tons and 40" tires - they are always in low range offroad.
If you plan to drive your big-tired, lifted, box at 80MPH, then:
a) you should already be planning on low MPG
b) the engine needs some revs to be happy with that load.