Newbie here. It seems mpg is all I care about and airbags.

 

Less features = less weight.
Longer gear ratios.

 

and the HX model only uses 2 cylinders when not accelerating

 

I also thought it did something with super lean AF ratios at cruise? like 17-18:1.... could be wrong though.

 

This is true in the 49 state legal HX models

 

My California model HX will enter lean burn as well. I don't believe it has been modified, but I am not the original owner, so I can't be certain.

Anyways, the reasons:
1) Taller gearing. The HX tranny has gears unique to it in the tranny and final drive. Mine runs about 2200rpm at 60mph.
2) VTEC-E. Similar to the EX, the HX gets VTEC except the lobe profiles are shaped more for economy.
3) Lean burn. Under certain conditions, the HX will operate under extremely lean conditions (Honda lean burn engines can run as high as 25:1 A/F ratios!). Wide band O2 sensor is required to manage this.
4) Weight. While not as light on features as a DX, it doesn't have many of the EX standard features. No sunroof, no power windows, no rear sway bar, and more.