2013 SCrew, max tow FX4, 3.73. 2 months old but we drive about 75/25 city/highway, get around 16 in town and if I can keep the pedal out of the floor mat I average 21 ish at 70 mph, 16.8 at 85 mph way out west here where we have 80 mph limits on I-15! Even towing with a 7460 dry weight toy hauler, then add 35 gal gas to trailer, 107 gal fresh water, 4 seat Kawasaki Teryx4, and camping gear for a 4 day weekend I was still able to pull off 10.2 mpg.
In my view the rear axle ratio is only a small part of the equation when it comes to MPG. You also have to take into the equation how you are going to be using the truck. Is it going to be a family grocery getting vehicle, is it going to be used as truck carrying a load, towing a load, city driving, highway driving and how heavy is your right foot.
If your going to be running pretty much no load on the hiway a 3:15 would net you the better MPG
If your running say a 50/50 mix hiway to city empty to lighter load, I would go 3:55
If running city more than hiway, or carrying/towing a heavier load I would choose 3:73 (which I did)
As for me with the 3:73 ratio I don't watch the MPG, I drive it as I feel in the mood to drive. I like the off the line in town TQ that the 3:73 offers me, I like knowing that the 3:73 gives me the ratio I need to pull a trailer and not suffer lack of power.
Also keep in mind what you may have planned for any future changes in tire size, my son had an Eco FX4 with the 3:55, then added 35" tires along with 4:10 gears which kept things close to where they were to start in overall ratio.
2013 S-Crew XLT 4WD, 3.55 axle, 5.5 ft. bed, Agricover soft tonneau cover, 18" factory wheels. I've not reset the trip odometer for the past 2800 miles; it reads a 19.5 MPG average. I drive about 60/40 highway/city. My best has been a 22.3 MPG average on a 200+ mile road trip; half of that trip on a highway at 62 mph, the other half on the interstate at 77 mph. However, I'm going to calculate the MPG by hand on this tank of gas to see how it compares to the computer's reading. I'm not certain the computer is entirely accurate.
Hi everyone!!! First post but been lurking for a little while. Love the forum by the way. 2012 xlt. 3.15 rear. Average 20.8-21.5. About 80% highway. 19,500 on the clock with a lifetime average of 19.9 due to pulling my bass boat from time to time.