yeah, back in the day it did make sense. trucks had crappy hubs, or had full time 4x4 or were constant motion systems.

Nowadays, everyone is so prejudiced about gas mileage that they automatically assume a 4x4 gets horrible MPG.

I can see a 1% loss from friction in the t-case and transfer of rotation from the transmission to the t-case to the driveshaft. there is 1 more transfer of power than the 4x2 has, and the gears are spinning in gear oil after all. but 5-7%...nah. that's a 15-21 hp loss.

Makes me think it's one of those "word of mouth" things. Sounds good until you really dig into it. Remember when guys used to put mothballs in the gas tank because it gave your motor instant HP? I'll admit, I did it on an old worn out Contour...it did work, but the car wouldn't run after that 1 tank of mothball gas.

 

Mothball gas.. man, I had that once for like 2 days after eating some weird 3 pepper dishes at a Thai restaurant..

 

My 2004 F150 Supercab FX4 with 33" tires consistently got 12 MPG the entire time I had it. It never got any better.

 

This might have been where I came up with the 5 - 7% loss?
https://www.f150online.com/forums/sh...driveline+loss
It is showing significant loss. I'm sure each case is different .
I dyno'd 214RWHP @ 5000RPM with 3"AF1 & SIDO Borla and about 1000 miles on the truck.