Alas, I wasn't able to do proper testing... And conditions were absolutely TERRIBLE:
1. It rained on the way home
2. I had to follow someone, both direction - someone that went really really fast (70-85mph)
3. Inconsistent traffic
4. Rain + speed made me loose some components
So about 150 miles highway.... averaging ~75-80mph... mpg=41.5. Some a/c to battle windshield fogging.... Also into a headwind on the way up to my destination.
When I dropped down to 55 for a few miles, I was getting 50, barely.
This shows that the cardboard isn't adding any frontal area:
After the rains - I needed to do some surgery. My average went down a little, but I used a lot more a/c.
Evidence of nothing really - due to my inability to do any proper testing :/ But I've never had such good trip FE
Lots of parental shock factor though