Not sure where your getting the numbers from but
http://www.fueleconomy.gov is listing the 09 civic ( don't have a 10 model yet ) as 25/35. OK scratch that I see you took the numbers from the base model which is 21/29 compared to the Impala's 18/29 which is really not an apples to apples since neither car aims for the same buyers, is the same size or uses comparable motors. A fairer comparison to both cars would be civic versus Malibu
One thing I will say ( and it has no bearing on me owning a Toyota Yaris ) is the EPA ratings still need to be taken with a grain of salt. Our Yaris which is rated at 29/35 averages out at well over 40 mpg per tank.
The Same can be said for my 2000 astrovan which is rated at 13/18 with a combined of 15 mpg. Generally I average out much higher when mostly highway traveling at 22-23 mpg and around town between 10-12. I drive fairly conservative speed wise around town and keep up for the most part with highway traffic.
I know you feel strongly about your impala. Much in the way I do about my Astrovan. Sure my Astro is 9 years old but Ive owned it since about 2004. Its been far from trouble free nor cheap to repair. 400.00 starters and 2500.00 head gaskets are more the norm. I know since I just spent over 4000 to repair many of these things myself rather then paying the dealer well over 6000.00 to perform them.
These prices seem more in line with my last Isuzu which was ridiculously over priced when it came to replacement parts and labor.
Theres good and bad on both sides now thats for sure.