I have ordered my trucks through fleet sales since 1991, not only do you get exactly what you want that way, but it usually saves me a lot of money, around $10K on the 2014 and 2020 trucks I bought. I usually get the mid axle ratio, in the case of my 2020 I got the 3.55’s, the choices were 3.31, 3.55 & 3.73
The 3.55 with an electronic locking axle would, imho be the best all around ratio. (I’m surprised that there is no limited slip axle available though.). On my 2020 F-350 CC SRW SWB 4X4 diesel I have the 3.55 locking axle, it will only stay locked up to 25 mph, fyi. Most of the time I do not need the locking diff at my boat ramps, Ive needed it a few times to pull the loaded trailer out, and a few times I’ve needed both the 4X4 AND the locking diff. The first time that happened I was very happy to have both.
The 3.55 is the best compromise between the 3.31 & 3.73 in regards to tow capacity and fuel mileage. The 3.31 will get you some better mpg empty, but worse mileage towing or bucking a lot of wind since the mechanical advantage of the engine is lower and will require more throttle when compared to the 3.55. The 3.73, while not increasing the towing capacity will give more grunt / engine mechanical advantage and ease of towing throttle wise than the 3.55 but will spin the motor faster and therefore use a bit more fuel empty and under “lighter” towing conditions, and arguably slightly better mpg at the heavier end of the tow rating, and will get you up to speed more quickly.
Another example might be my high altitude impellers, they have less pitch than the oem impellers so they pump less water at a given rpm, but the engine has more mechanical advantage and allows the engines to reach max rpm’s where the horsepower peak is. The performance is much greater at the cost of roughly .3 mpg at best cruise because the engine has to spin more times to pump the same amount of water (stochiomatic) which is a great compromise as my boats performance at 5000’ on a hot day was lack luster, and now it’s much perkier.
Here‘s something else to understand about turbo charged engines. The more load you put on them the more efficient they become, as you are burning more fuel and creating more exhaust gas which in turn is creating more boost. I had a great example of this coming home from a race in Idaho one year, I was pulling my toy hauler (12,500#) into a 40 mph head wind at 65 mph with my 2005 6.0 diesel, I was at roughly 80% throttle making max boost and thought my mpg was going to drop into single digits from the normal 11.3 mpg. To my surprise I was getting 10.4 mpg.
I’m not sure if the gas motor turbos are variable vane turbos like the diesels. If you keep the boost level below a certain point, in my diesel that is 20 psi (roughly 40 psi max), the mpg is the best while towing at 65 mph, I’m sure the gas motors are the same. Also, if you can keep the rpm at the beginning of the peak torque curve, my truck is 1600 rpm, you keep the stochiomatic fuel use as low possible when towing. It’s a balancing act that I’m sure you’ll figure it out pretty fast using the instantaneous fuel mpg while you are motoring along. The turbo boost (now Kit) is also dependent on the rpm of the engine due to the volume of the air flowing through the engine, that stochiomatic thing again, so at a higher rpm the turbo is spooled up and making the most boost with the least amount of fuel. Its a bit of a learning curve with a turbocharged engine to get the most out of them as far as driving style, but you’ll get it figured out quickly. One thing for sure, the eco boost engined F-150’s are rockets! I’ve driven a few, and if you stick your foot in the fan they haul ass. And if you’re easy on the throttle while cruising empty they get great fuel mileage.
