My 92 dodge has 4.10's with a getrag360 similar to a nv4500,, its great towing.. but top speed with 31's is about 80.. and to get over 20mpg you have to stay about 60mph anything over that and the mpg starts to fall. here is a good gear calculator
Gear Ratio Calculator
Try and keep your cuise rpm under 2k, the motor is most efficent at about 1750rpm.. mine worked best as a daily driver with 35" tires but towing with them sucked due to sidwall flex.. I tow heavy 12- 15k..
Now my crew cab has 3.54's in it but alot more hp, so i just tow in 4th but also can cruise empty at 70mph and still get good mpg it has 32" 225/70/19.5's on it..
use the gear calculator and try and work a combo that keeps you under 2k rpm towing and cruising.. good luck..
also if you are looking for a mileage combo with small tires the early non overdrive dodges came with a dana 70 front and rear that had 3.08 gears in it.. works great in a 2 wheel drive with small tires and a overdrive... 24-25mpg consistanly on the highway.. rember that larger-wider tires worse the mileage....the 35's cost me 3-4mpg..