If I were only getting 23 MPG, I'd be so hopping mad ... here in NY, I'd bring a "Lemon Law" case to court and try to get the car replaced. Given that most of the FEHs seem to do so much better than that (my last tank was 31.5 mpg, still going up (thanks to the nice weather, and perhaps breaking in - I have less than 2000 miles on 'er so far)), it might pay to go for a replacement (rather than asking for your money back, which would be a long shot). I know that low mileage doesn't usually give rise to an actionable claim, but for a car like this, where such "out of range" mileage is probably a symptom of an actual malfunction of some type, the technology is so new, and mileage is one of its hottest "selling points," you might have a chance, particularly if all you want is a replacement.

Have you inquired as to your possible legal remedies in whatever state you're in (besides a state of exasperation, that is)?

For that matter, have you asked the dealer if it would consider replacing the car, or perhaps giving you a "loaner" FEH for a couple of weeks so you could see if it is something about your driving or the terrain, or truly a problem with this particular vehicle?

Just a couple of ideas, forgive me if you've been over this before.

C.