I haven't been around these boards enough to know what people's real world experience with the mileage is on their 6 speed conversions, but I just got 26.3MPG going 72 mph with the windows down the cruise control set and the radio blasting (read as no AC) with my 5 speed MT on an AWD . . .
We started the day at 1100 feet and ended up hitting 7400 before refueling at 5000 feet or so. I'd say overall the day had about 8k-9k of climbing. We even had 15-20 miles of tooling around a state park at 30mph and 15 min of idling in line to pay our park fees.
In my limited experience the A.C. saps up 3-4 mpg off the top of my fuel econ. Hell, I got 18mpg hauling 2500 lbs up the west virginia mountains at 70-75 mph a few weeks ago (sorry E, it won't happen again - that wasn't very nice of me, but you only move out west once, or twice in my case.)
so here is the "thing" about a six speed mod (remembering that I don't have one, nor do I have any real world data from anyone who does) it doesn't do anything to help the stock engines abysmal torque curve. Peak Torque isn't really something that honda has traditionally cared about (my snap judgement looking at their factory numbers passively over the last few years, and driving an AWD Audi with an engine tuned for usability - damn I miss torque).
So what we have here is a car who's peak torque kicks in around 3500 rpm (and the VTEC pops in at 4K?). which means that in factory 5th gear you are traveling at ~71 mph when you hit that peak torque. Much faster than that and you really have physics working against you. The E has the aerodynamics of a cardboard box (which is why you can fit so much stuff in one), and the faster you go, the more that works against you.
So having the engine running at 3k rpm at 75 helps in the world where the car isn't so damn hard to push that fast, and the engine isn't noticeably weaker (a world that doesn't exist/ costs an additional $10k for a super charger - which is pretty sick if you ask me.)
my hair brained idea is to drop a VW/Audi TDI into the E when this engine gives up the ghost (or I get $10-$15K to sink into a car that is over a decade old.)
Heck, while the engine is out I could even spray the thing Whacky Khaki.
I'm guessing that after all of that I could probably push 30mpg at 75. 260 lbs torque would be nice at 2500 rpm. The extra 4 HP are just a bonus.)
edit: the max torque on the E is at 4500rpm - which is laughable