i fig'd, at 18k miles per year, my 5.3 at 17.5 avg vs. the diesel (which I am getting in a few weeks) ~24.5 avg, well, that's an 8 mpg difference (we do a lot of highway miles, I am expecting the diesel to do better as an average than that, based on the anecdotal evidence posted here and other places). That is 300 gallons extra per year for the 5.3. Which, at todays prices, is 1200 bucks. If I end up paying an extra ~$1/gal for the diesel, I am still ahead by ~$500.
A few months back, diesel was 0.3 cheaper than premium, right in the middle of regular and premium. Ay times (like a year or so ago) it has been 0.1-0.2 cheaper than regular (PNW). I am hopeful it will move back in that direction, but even if t doesn't still seems like a good decision. Nonetheless, I am shocked about how high diesel has popped in a few weeks.
And while I did move to the diesel partly for the mileage, I also went for the engine. Both sides lifters replaced (along with multiple other problems with this vehicle), I thought a change was for the best. That said, the mileage I have been getting n the 5.3 has been pretty good - way better than my old Pathfinder, and almost as good as the old Odyssey.