Couple ways to look at this, as driving conditions and style will affect the outcome either way.
On a completely flat surface at a steady speed, say 75 mph, comparing two identical trucks, with the only difference being gear ratio, the lower gear will get better fuel economy as the engine is turning a slower speed. This is a general reference not factoring any extenuating circumstances.
However... insert variables into play and things change very rapidly. The following are a few scenarios as such.
Around town driving mostly - this removes gear ratio from the equation for multiple reasons. First off, higher ratios require less effort and pedal force to accelerate. 30% throttle may give you the acceleration you desire with a higher gear, where as you may need 50% to achieve the same with a lower gear ratio. Second, you generally never sit in the overdrive gears long enough to notice the economy difference in town as you don’t achieve speeds requiring such gears. Third, when factoring in hills and terrain other then flat, the engine must downshift to the appropriate gear to maintain speed where as with higher gear ratios, often it will leave the truck in overdrive or higher gears.
Towing - should be self explanatory, but with a heavier load behind the truck, the lower gear ratio will require downshifting and more pedal force to maintain speeds or accelerate, where as higher gear ratios do not, and often can maintain speeds even up hill in overdrive.
Higher speed cruise, hills, mountains, wind resistance, lift kits, large tires etc... same deal here, once you reach a point where the throttle has to be opened substantially to maintain a speed, or accelerate to your liking, the higher gear ratios are going to have an advantage as it will require less effort to maintain, even though it may be at a higher rpm.
Low gears are great if you drive across country, or on long distances often, or drive responsibly, but for any other conditions, I feel higher ratios are always advantageous.
For what it’s worth, I bought my truck in Tucson and drove it back to Albuquerque through flagstaff (stopped in Phoenix) and maintained 85-90 mph the entire way, including through the mountains, and still averaged about 17-18 mpg for the trip, with the 3.92 gears. I left it on cruise and didn’t have to touch it through the mountains unless I got stuck behind trucks, and I imagine had I had the lower ratios, I likely would have been hot rodding it much more to get the speed and acceleration I needed, worsening the fuel mileage.
If there really isn't a difference in MPG, then why is FCA offering the 3.21 at all? The logical side of me says that they had to have a reason to offer the 3.21...but what is that reason?
