Okay, update time. I got 285.1 miles before the fuel light came on.

Filled up, 36.6 gallons in until click off. After which, the nozzle wouldn't pump anymore, actually felt like it had broken. That said, gauge reads full.

My maths says this tank yielded me 7.8mpg.

Not bad, in comparison. Still not great though. Can I chalk some of this up to snow covered roads and using 4WD (or locked hubs even in 2WD) a lot?

Not to mention the -10 weather we've been having recently...

 

Much better.

Now, just to be on safe side look under fuel tank and make sure there isn't a leak when full. A hole high up on tank could only leak when tank is full.

Tire size...if your tires are larger than oem than your miles are off. 34" tires and you need to add about 5% or so to your miles.
Next I would check brakes and hubs to make sure nothing is dragging.

Then I would check for leaking exhaust manifolds. Any exhaust lost via a bad gasket will make O2 sensors read lean and the pcm will richen up air fuel ratio causing poor fuel mileage.

Then check pcm for any codes.

End .....for most as code check might tell you where to go next.

I personally would also put in new plugs, coils, MAF, IAC, fuel pressure regulator, and get refurbished injectors too. I know that's about $500 to $700 in parts but that just what I would do. I know everyone can't do this.

On a side note, is your thermostat a 185 degree unit? Being in a cold climate I suspect you may have a hotter tstat? This could, I'm not 100% positive, confuse the pcm as to how it calculates the optimum air fuel ratio.
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3.73 gears with stock tires. I've been toying with going to 4.88's actually. What size tires have you got? 37's?
I've not run this truck in any weather other than snow yet (it is that new) so I'm holding off rash decisions until it's run on clean roads and warmer air. I'm happy to hear I'm not too far off what you've seen in winter though.

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You are dead on for my winter MPG. I'm between 7 and 8 MPG as low as the 6's if it's cold. That's running in 4x4 100% of the time and considerable idle time. GPS verified.

I also have the same issue with my gauge not reading correctly. I've had 3 V10 Excursions and my current one is all wonky. My other two it was very clear 1/4 was 10 gallons gone, 1/2 was 20 gallons gone, 3/4 was 30 gallons gone and E left me with 4 gallons, every. single. time. My current one turns the gas light on, drops the gauge to E and says 0 miles to empty when I can only pump 36-37 gallons in it, very aggravating as I've replaced the fuel pump and sender trying fix the problem but.

 

 

With 35" tires you are about 11 to 12% off on miles driven.

So your mileage is closer to 8.7 mpg

still not as good as it should be. I average 11.5 on my supercharged X with 4.10 equivilant gears and 13 mpg in my other 2 excursions with 3.73s. And that's at 8000 ft elevation.

Much better than 4.5 mpg though.

 

Sorry I mistyped!

I meant to put I am thinking of getting 35's down the road.

I DO have stock tires. 265/75/R16 @ 31.6"

 

 

 

One of the "benefits" of so many cylinders.
But you should still hear the difference of intentionally shutting one off. Or unplugging one and NOT hearing a change!

 

 

​​​​​​Interesting. It would be my first time doing something like this, are there any good videos on it? I know it's simple enough, but watching others is how I learn best.
Also, my truck runs OBDII software. Hood confirmed. Will a scanner detail this easier for me than the unplug test?
I've also bought a 22-inch screwdriver to do the injector listening test. I'm sure my new neighbours will think I'm mad!
Thanks guys - your help is amazing.