F150miz

Im going to try not to beat a dead horse here, but I just bought a 2014 XLT crew 3.5 ecoboost with a 6" lift and 35" tires. It has a 3.31 rear end and I am looking to go up to either a 3.73 or 3.90 for a little better towing and get some power/torque back as well as MPGs.


My questions is, I can read the gearing charts and 3.73s with 35's are very close to the stock tires with 3.31. But what is the ideal RPM for MPG? I travel 60 miles a day on a 55mph and average 60mph. What RPM would I want for the best MPG and what could I expect to get? I know everyone says below 2k RPM, but I have to imagine if they fall too far below 2K RPM at 60mph, it might come close to the shift point in the trans and id be travelling everywhere in 5th...


Any and all input is appreciated, id just like to get better than what im getting now (12.1mpg) and get some power back for fun and torque for towing (boat and trailer is only 4500lbs).


Thanks

Apples

But do you cruise to work (traffic permitting) at 60? or 65-70?


I'm new to modifying late-model F150s but not new to general principles of automotive performance, efficiency, tire size and gear swaps... just out of the lo0p with this new range of larger tire diameters, so in that regard I'd have to refer to some charts or do some old school math... bear with me while I go random on you.


Off the top of my head I think you might be better served with the shorter of the two ratios you mention. Another thing to consider is the weight differences between your truck's OE wheel and tire combo and what you're running now. A heavier setup will affect (in a small way) your fuel efficiency from a standing start in addition to the load created from the taller effective final drive you have now with the 35" and the 3.31s.


I suggest the shorter gearing because your engine has hair dryers hanging off of it, and the load created by a taller (effective) final drive has you tipping into the gas just deep enough to keep them feeding the motor, rather than having them just spinning along for the ride (best fuel efficiency, but there has to be no load on the truck, including no head wind and/or no uphill grade)


The 2000 rpm figure is another generality... I don't know if your truck has that instantaneous fuel mileage readout (which can drive you crazy), but if it does you might gauge the best cruise revs vs. throttle position once you re-gear your axle(s).


Hope this makes sense... yeesh.


:-)

Eric Kleven

Did you get this figured out?
I might not go with gears any 'faster' than your stock tires with 3.55 rears. If you're towing, maybe lean more into the 3.73 region. With those big tires, putting in a set of 4.10s may not be a bad idea.

If you want to get into some silly math, you want your average piston speed to be between 5 and 6 meters/second at cruise.

;-)