I have a 2005 F-250 SD Short Box with a 5.4L V8, 5-speed Allison transmission and 4:10 rear axle. The mileage on it is only 27,000 miles. I have sold my 5th wheel and would like to keep the truck. I would like to replace the 4:10 gearing with 3:55 gears, or even 3:31/3:15 gears if doable/workable.
The cost at the dealer would be approximately $2000 to make the change (parts, labor, taxes, etc). I'm not concerned about the cost as such, but only if it will increase the MPG. This vehicle would not be a daily driver but used when I want to take a trip. I hope you get a chance to read this considering all the mail you must receive.
Not sure how long it's gonna take you to cover the $2k with cash saved at the pump, but sure, dropping the rear axle gear ratio will increase fuel economy -- probably more so on the highway. Of course, the new gears will also reduce your jump off the line and towing capacity.
I suggest keeping the truck semi-stock, meaning replace the gears with a ratio that was available during production of that specific Super Duty platform. This can help in two ways.
A biggie, if you go too low of a gear ratio you won't be able to get out of your own way. Also, you have to recalibrate the Powertrain Control Module with the new axle ratio to keep the speedometer and other engine control characteristics in sync. By sticking with factory gearing, the techs at the dealership should be able to make the software adjustment. This can save you the hassle and expense of needing an aftermarket programmer to go outside of the manufacturer's parameters.