Jim,
Lean burn is the trickiest part of this testing, and I have an unconventional solution. Rather than trying to stay in lean burn, insist on staying out of it. You may only get 55mpg (
), but if you drive at 75mph, FE will be less twitchy. Also, aerodynamic drag is easier to measure at higher speeds.
Another option which I think you'll like better: Instead of measuring fuel economy at constant speed, measure speed at constant fuel economy. Nail your mpg meter to 80mpg (or whatever number), and hold your throttle foot steady even during NOx purge cycles. Every ten mile markers, record segment time and mpg.
I've recorded a few coastdown runs by videotaping my speedometer. Unfortunately, I haven't yet done a bidirectional, high/low speed run, and I haven't gotten any good numbers out of my data. I'll be out in the sticks this weekend, so I will try to make time for testing.
I might actually have access to an aerodynamically stock Insight for a little while, starting in a week or so.
In which case, I will attempt to verify the 0.25 figure via coastdown.