You'll generally see the most efficiency out of an ICE between 45-60mph (you can find reference to that
here , for example). I'm talking Otto engines here, not Atkinson.
If you have the Navi & it's instantaneous FE display, you can easily see (
on level ground) how up to 70mpg can be held at 45-50mph, 60mpg @ 55-65, and 50mpg from 60-70. After 70mph, things start dropping fast.
As far as lower speeds - I'll bet that your best range is between 42-50mph. 42 is where the AT shifts into 5th (and will hold it down to 35mph). At 48mph, the torque converter locks up (it will hold this down to about 42 or so). I notice on my instantaneous that I easily gain another 10mpg after the shift at 42 (and I expound a lot more energy than I should figuring if I should go to 5th gear in a particular situation, and then trying to get to 42, while still having some distance to use it effectively before my next stop, I might add...
I can still see 50-60 mpg on speeds between 35-42 (this is in 4th gear, not shifting to 5th and coasting down), once VCM is active. Before VCM activates, 40mpg is the best I can personally coax out of the instantaneous.
BTW - your figures from the trip odo seem to jive fairly well with mine from the TC - you may see another thread or two comparing the 45+ that Xcel got with my car on a 160mi highway trip, with the 44+ that he got in his brand-new L4 Accord on it's initial trip from the dealership to his house (about 900mi; long story
. Bottom line is that both of us think the HAH can do better than 45 under the right conditions; Xcel thinks 60+ mpg can be had; I'm thinking in the lower to mid 50s. One of these days, I'll figure out the flattest 600mi stretch of road I can find (I-80 through IL / IA is a good, but very boring candidate), and head for it. Or maybe I'll have Xcel head for it...LOL
If you can, try and maintain those speeds for about 5mi or so, and see if they don't bleed off a bit. I used to do these types of resets on my trip odo, before I learned the value of the instantaneous, and I'd see "46" become "37" within 5 minutes or so. Of course, I probably wasn't driving with load back then, either. These days, as long as I'm on a highway, and don't run into traffic, I just keep seeing the odo keep
climbing - which was happening during the two best highway trips in the car to date. In the one case, Xcel ran out of road (we arrived home). In the other case, I ran out of 'good' road (washboard so bad it made me hop in the seat).
Regarding the 'eco' light - VCM is documented to cut off at / above 82 miles per hour. It's effective range is 20-82 miles per hour (although I can get it to initiate at 18 miles per hour, per the speedo, and maintain down to almost 15mph, but the engine will lug below 18mph if VCM is active)