[christofoo]

Random thoughts:

• Considering your mileage, I'd expect you'd find repair costs significant also, I'm really curious how CNG compares vs Gas in per mile maintenance, and I think that should weigh on your decision. (I find for my ooold Corolla at 45 MPG that gas and repairs cost almost the same, each in the $0.1 per mile range, although I expect my repair costs to come down since I'm doing more things myself now, and I think that Honda's and Toyota's are mostly in the $0.05 per mile range for most of their life (prior to 200k)).
• (I'm just guessing, if you did a CNG conversion on the Impala that the Impala would end up with lower throttle loss and higher efficiency, corresponding to the reduced HP, so your overall savings with CNG may be noticeably higher than you'd predict just comparing cost per unit energy. If this is true, it would also open you up to more significant benefits available from aero mods.)
• Can't seem to find front-seat specs, but if you want to look at replacing the Impala in search of better gasoline MPG, I believe that Ford Escort's are noticeably more roomy than Honda Accord's or Toyota Camry's, but are comparable in other ways. That being said, my personal belief is that Ford's cost more to maintain per mile, particularly in comparison with the Honda.
• Also if you go in search of a different car, if you can put up with 2 doors, my impression is that coupe's usually have more aerodynamic rear windshield areas, so (short of a boattail), you can probably get more from hypermiling and modest aero mods in a coupe vs sedan or hatchback, especially if it's a manual.
• EDIT: Even if you find that you can tolerate a mid-size car, considering your mileage it may still be favorable to do a CNG conversion. (I'll leave you to do the math though. )