Re: Dually gearing
SRW and DRW run different tire sizes (overall diameter is slightly taller with the SRW) so by going to a lower gear set (if you have 4wd, you'll need to change both axles) won't net you that much change in RPM at any given speed. The different between 4.10 and 3.73 (knowingly what your friend has) isn't really worth the $750 or so it will cost to change the gear sets from what you'll save in fuel. If you drive 15K miles a year it'll take you a good 5-7 years to pay for that.
The downside, 4.10 is what you want when towing several tons of anything. Taller gears allow you to drive in a higher gear without lugging the engine which can thus cause erratic shifting (excessive heat can cause transmission failure). If you can tow in top gear (overdrive) now, you may find yourself pulling the trailer in a lower gear not only to prevent gear hunting, but you will have minimal throttle response when you have to climb even a 4% grade, and will need to downshift or lose speed. In this case, mileage will be worse.
If you don't tow often and drive the truck empty on a somewhat normal basis, 3.73 gears will save a bit of mileage if you prefer to drive over 65mph on a regular basis. If you drive mostly in town empty, the 4.10s will be the ones to have as lower gears will get you up to a set speed quicker and thus save mileage.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
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