Hybrids are engineered from the ground up for maximum FE. Honda wouldn't have put that spoiler (or anything else) on if it decreased FE by any amount whatever.

I believe mwong99 is on the right path.

In pickups (according to Mythbusters), it gets better mileage and less drag with the tail gate up - because it reduces turbulant air.

It probably helps on HCH-II, but I tend to think that it was for visually distinguishing HCH versus normal Civic.

I think its like a Formula 1 car. It produces enormous downforce. Which means you don't have to lift when negotiating turns.

"HE HE HE"

Howard

Honda's marketing strategy seems to be NOT to make a distinction between the hybrid and the regular Civic. If they chose otherwise they could have made the HCH much more distinct.

The spoiler is meant to increase FE. I have no doubt about that. For comparison look at the Prius. The older model had a spoiler. On the newer model they changed the design so that the tail end is very high and drops straight down. So there's no longer a need for a spoiler. Although it may be counter-intuitive, it seems that a higher tail with a straight cutoff is aerodynamically better, and a spoiler helps accomplish that.

2006 HCH Alabaster Silver w/Navi
2003 Honda Accord LX