I agree that I'd like a bit less rpm on the interstate.
Four ways to achive that:
1) another gear in the tranny. That's up to Honda; cost prohibitive and design intensive to the nth degree for us mere mortals.
2) taller 5th gear in the tranny. See above answer.
3) taller rear drive ratio. A bit more "doable" from the design and machining perspective. It's done in the aftermarket all the time for performance cars and 4x4 trucks. Perhaps enough positive pressure for the 'Wing crowd would persuade them for a design quest. For those who don't think this all the way through, it would be quite intensive as there is a pinion AND ring to address. The upside is of course, better economy. The downside is a bit slower acceleration relative to any given gear selected in the tranny.
4) taller tire. Been done several times before. Drop a few rpm, and gain some ground clearance at the same time. Some like this approach; some don't. Another upside to this approach is that it's easy and inexpensive (realtive to the other options) to do, and it's easy to reverse if you don't like it.
As for the "amount" of rpm drop desired, that's a debate within a debate. My observation, from both GL1500's and my current GL1800, is that 350 rpm or so would be good. I say this because there seems to be a universal aspect of fuel consumption with the GL series of engines. It is true of all internal cumbustion engines as well, generally. Spin it higher, consume more fuel, right? I don't know why, but the GL's seem to hold fuel consumption fairly steady until around 3000 rpm, then they start on some parabolic P+I BSFC consumption gas guzzle fest that just goes up at an insane rate past 3000 rppm. If you go from 3000 rpm to 3300 rpm (10% gain), you might use 20% more fuel. And it gest worse from there.
Now part of this is because 3000 rpm is APPROXIMATELY 70mph, depending on the speedo accuracy. Most people don't realize that wind resistance is not linear; as your speed doubles, the wind resistance is SQUARED! In other words, to go from 35 mph to 70mph makes the wind resistance FOUR TIMES the amount at 35 mph! This is why the harder you push past a higher speed, the fuel consumption rate goes way up.
The upside is the the GL1500 and GL 1800 engines have enough torque that it's quite possible that they could offset the energy requirement to break past each successive mph increment. At some point, the engines would get into a "zero sum gain"; if you dropped the rpm by 25%, the engine may not have enough torque to overcome the load based upon speed and wind. That's why I think an approximate 10%, no more than 15%, drop would be enough to gain fuel economy, but not enough as to overburden the engine.
Let me put some rough numbers to it. Assume 70mph is 3000 rpm in a traditional GL1800. If you went up 10% in speed, you'd be around 77mph and 3300 rpm. If you could drop the rpm by 10%, you'd be back at 3000 rpm; at that point, I feel the GL1800 has enough torque to overcome the load, and the drop in rpm would be more than the "throttle" needed to sustain the rpm. You will NOT get a direct 10% gain in fuel economy, as it does take some more fuel to sustain that torque load. But you might gain 5% more economy?
Engines are most efficient at peak torque. I'm not sure where peak torque is on a GL1800, but it seems to be around 3000 rpm or so, by my general observations. The goal would be to get the most vehicle speed out of the engine, realative to the rpm and BSFC. If I can get the GL1800 to turn 3000 rpm at 77 mph, rather than 70 mph, I gain fuel efficiency, as long as the wind resistance load does not overcome the BSFC relationship.
Food for thought. The next time I'm going to change tires, I'm going to try a taller tire. The rpm drop ratio is about what I'd be looking for. I didn't buy the GL1800 as a drag bike; it's a LD touring and commuter ride for me. If I gain 10% economy at 75mph, but only loose .3 seconds in a 0-60mph run, I'll take the fuel economy edge any day, especially with gas nearing $3.50 a gallon!