I know that when the truck encounters slick conditions and adds in the front axles as driven that the mpg is going to drop a bit due to all the added rotating bits.....but on dry pavement, does leaving the truck in 4A affect (lower) the mpg at all?
I'd like to just leave the truck permanently in 4A all winter, but the mpg seems to drop when I do so....could just be me.

 

Anytime you use 4 wheel drive in any form it will lower your MPG.

 

Yeah, I know -- but on dry pavement, the truck is still basically in rear/2WD. What is turning/rotating in that situation that will lower your mpg?

 

I don't know if it puts the tcase in 4hi and leaves the hubs unlocked or if it leaves everything disengaged until it slips, but unless it is changing something (which I'm pretty sure it does) it won't impact mpg.

Spinning a tcase, driveshaft, and axles takes power. The only thing left would be locking the hubs to spin the wheels.
If it just puts the truck in a ready state but doesn't shift anything, it will only burn more gas when it engages.

If the conditions are bad enough for you to consider 4A - you shouldn't be worried about mpg. Just do it.

 

Good point.........it's just that sometimes I forget when the weather does get better to rotate that little **** back to 2WD.....it's going to have to be one of those things that I pay more attention to, right?

 

Aside from all of the above Ford advises not to run in four wheels on dry pavement, at all.

 

I can shed some light on this. When in the 4A mode, the front hubs are locked so you are turning the front differential all the time, even on dry pavement. When wheel spin is detected, something in the transfer case engaged the front prop shaft (a clutch of some kind probably). So YES, having it in 4A does lower mpg. Want to see how? Leave the truck off, and jack it up and spin one of the front tires. You'll feel the resistance of turning the front differential. Now start the truck, put it in 2H, and now spin that same wheel, BIG difference. There's your mileage drop.

 

In the 4WD setting, that is true -- but that is what the 4A setting is for, when the pavement is dry, then icy, then dry again....the truck decides when it detects the rear wheels spinning and engages the 4WD, and there is no chattering on slow turns in 4A like there is when in 4WD.

 

When you are in 4A the truck is engaging clutches and when the rear wheels spin the clutches engage the front wheels. So there is a drop in MPG when in 4A because the truck has resistance from the clutches.

Here in AK I run 2wd unless I am unsure of the roads or if I am pulling up to a stop light and it is icy. I swap back and forth as needed. No biggie.

 

and trust me, you won't forget to switch back to 2wd if you leave the truck in 4WD on dry pavement! First corner will remind you!!
I barely ever use 4A, only when my wife is driving in the winter. Where I may switch back and forth between 2wd and 4HI, i just put it in 4A for her and let 'er go!