23-25 is absolutely phenomenal! My rig gets 13-14 driving relatively fast with big tires and 3.55 gears. I hate the mileage and love the pickup. I've never averaged better than 16. I do live in a hilly area.
The 6.9's seem to get better MPG's than the 7.3's. Hopefully it comes up and up for you. I posted in the "So, what did you do to your truck today?" thread-- if you are seating rings that'll affect the MPG's for a while.
And yes timing can affect MPGs a lot. But sometimes, not so much, like in my engine. I dunno what the heck is up with mine but she can be 6* either way and still get about the same MPG. That's no exaggeration, I have a meter!
That said, get a meter, they are worth it. Plus then you can help others that need timing. I sold some N/A stuff to a nice man that was just getting into these engines. Ended up timing it just to be kind and he was blown away with how much better that thing ran. Power, MPGs, sound, drivability...he loved every minute of it.
That also reminds me that if your timing is quite retarded, that will help it cold start a lot. I once had 0* timing and my stock engine could cold start without glow plugs at 60* ambient. Which is bonkers for anybody not named KansasIDI
But then I timed it correctly and it needed glow plugs.
