Tank mileage has been declining since the weather started to cool off and I've been fighting that. My wife does a vast majority of the driving outside of holidays so the past few tanks have been all her. Over Thanksgiving I had a change to stretch Gaptooth's legs on the highway for the first time since my belly pan additions, grille block, block heater and RE92's up front. The result was a really great tank:
I averaged just under 65mph, mostly using driving with load. The additions to aerodynamics and change in tires has resulted in my being able to stay in lean burn (75mpg) at 65mph. Previously I came in around 65mpg at 65mph, so my modifications have been worth an approximate 15% increase in fuel economy while on the highway. There's also the weather factor, with the air being cooler now, so in an apples-to-apples comparison it might be a slightly larger improvement.
Some interesting tidbits:
If you have the hazard lights button in the dash unplugged, the turn signals don't work.
If you have the climate control module unplugged, auto-stop doesn't work.
If you have a clutch-in switch engaged to prevent IMA assist/recharge, lean burn does not engage.
Driving on a concrete highway with sound barriers in moving traffic, for me, provides a very significant increase in fuel economy over fresh asphalt out in the country. On i-275 coming in to Tampa I was able to achieve an average of
101.7mpg driving at approximately 55mph over 8 miles, and on i-10 coming in to Jacksonville I was able to achieve
97.0mpg over 26.6 miles averaging 57mph. These were level roads. My best guess is that there's a "wind corridor" effect due to the traffic and sound barriers, and the road itself is significantly less sticky.