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Although the 5th gen Civic 4-door sedan has a Cd of 0.31, the shorter, blunt-tailed 3-door hatch has a higher Cd of 0.36.
Even though my car has a DX engine in it, the transmission is the taller-geared unit used in the VX and CX, so the EPA figures for the DX wouldn't apply. I found that the DX engine (installed when my car was still otherwise stock) has a lot more punch at high rpms than the stock CX motor, but didn't reduce the mileage as long as I minimized my visits into high rpm territory. So the mileage increasing effects of taller gearing trumps the mileage reducing effects of higher power, at least with the Honda motors that need to reved high for max hp.
This past summer, with close to 100,000 additional miles on the motor, with newer and a different brand of tires, and E10 gas, I was getting 85 to 90mpg on a steady speed cruise at 65mph on a level road. This is down from the 90 to 95mpg I used to get on my previous set of tires, straight gasoline, and a less aged motor. I'm not using any hypermiler tricks when driving at a constant speed on a flat road, those are reserved for urban/suburban driving and when driving on hilly roads. I've never gotten any mileage improvement with the pulse and glide technique on flat terrain on the few times I've tried it and as a result don't use it except in city driving or hills with their unavoidable stops or downhills.
Like Daox said, aero effects are most significant at highway speeds. What my aero mods have done is to let me get the same mileage at 65mph that I used to only be able to get at 45mph. I didn't have a SuperMID installed in my pre-aero days, but I'm sure if I had it installed at the time, I would have had no problem getting 95mpg while driving at a constant 40mph on a level road in the summer. I certainly don't have a problem doing that now. What my aero mods have done is allow me to get about the same mileage through a wide speed range from 30mph to 65mph and then rolling off at 70mph and above, as opposed to having the mileage start to roll off above 45 mph back when the car's Cd was stock.
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