The D15Z7 can get better FE than the D15Z1 in only one condition. Suspendedhatch wants more power but still really good FE, which for some, let's say living in a very urban area with bad drivers all around you, is a good goal.
It will always have more power but not always better FE.
I did an oil change before the trip. Mobil 1 full synthetic 5w20 and a K&N oil filter. I also installed the Rotas from the Integra. The lighter wheels improved ride comfort noticeably but the wider, softer compound tires no doubt negated any potential increase in FE.
I wasn't able to match the previous record of 47.5. The radiator fan was on constantly and I was running coolants temps of 220F+. All the bugs from these long trips finally pushed this 16 year old radiator over the edge. At 235k miles retirement is overdue. Not to mention that air temp was 85-107 degrees and that includes night time temperatures. Hopefully I'll get my new inch thick radiator today.
Car was loaded down with passengers and luggage. Moon roof was propped open as usual and the windows were down. No special driving techniques other than DFCO.
I finally put an A/C belt on but we didn't use it because it needs to be recharged apparently. Now I need to find someone that will recycle the old R12.
On the way home I came up with a theory for my very light load misfire (only happens at 80mph downhill). The ACCEL fuel enrichment is constantly triggering because it's so sensitive. At very lean mixtures with very little air present, a small, erratic increase in injector pulse width is probably a big deal. That could be what's causing the boiling exhaust sound and light bucking. I guess it's time to learn Accel.
Considering you're in a VX, shouldn't you be getting at least 57MPG?
No. VXs do well, but not typically that well. If you aggregate the gas logs here you see that the overall average is something like 45 mpg. Certain people never beat 50 mpg, and most people never beat 60 mpg. Only a handful of VXs in the garage have beat 57 mpg more than once or twice, and only 2 VXs in the garage (out of about 25) have a 90-day average over 57. So your expectation is too high.
Also, this car was heavily loaded and going fast. Altitude may also have been an issue.