- I have only owned the 3.31 Electronic gears, as I have ordered the FX4 package on my 2010 and 2015 F150 V8 and again on my 2018 F150 PS 3.0L V6 Diesel, so I cannot answer your questions.
While I understand your yearning to learn as much as possible before pulling the trigger on your purchase, your question is somewhat a loaded question, as the only person who could realistically answer this question is a dealer who has put enough demo miles on each ratio or an owner who has owned both
3.31 and 3.55 gears (read 2 different vehicles) in the last 15 months = I'm not sure any of us here can afford that.
If you are hung up on axles, I would review the Axle Availability
chart on Page 28 of the 2019 Ford F-150 Order Guide
to find that there are only three (3) axle options available for the F150 PS 3.0L V6 Diesel in only two (2) different ratios: 3.31
(See screen cap below)
Larger ratio = gears spin slower for same engine RPM and transmission gear = better gas economy
=> Based on the averages that
and I (both Platinums) posted above to your initial question, the 3.31 gears are costing me ~1 MPH is both city and highway driving
=> There are other factors involved in MPG such as: 2WD vs AWD vs 4WD, Active AWD (front axles always spinning) on Lariat and above trim levels, weight/payload, etc
Multiple owners have chimed in on other threads that they are towing small trailers with both axle ratios w/o any issues.