"From my observations what makes these vehicles so fuel efficient? 1) Lower HP"
This shouldn't be oversimplified. For example, compare the VX and CX ('92-'95). The VX has 31% more power, and still gets 13% better mpg. Why? Because the CX was built to be cheap to buy, and the VX was built to use less gas.
The VX has a bunch of subtle tricks to boost FE, like roller cam followers to reduce engine friction.
A good technical listing of various VX FE ingredients is in this
pdf.
"i own a civic VX and I have to say its not that bad."
The VX has 18% more torque (and the same weight) as the Fiat 124 Spyder I used to drive. That was considered a sports car. What's happened is that our expectations have changed. Trouble is, we can't really afford our new expectations.
suprapsu 
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