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From 2005 to 2020, I drove and commuted year around in a 1992 Honda Civic (Aerocivic) that I extensively modified to reduce its Cd from it original 0.34 to a low of 0.17. Here are a couple of things that I noticed along the way:

The mileage and range improved, rising from its original high 40’s mpg summer, low 40’s winter to low 70’s summer, high 60’s winter. Range increased from the low 400’s to over 700 miles. Maximum speed increased from around 100 mph to 140 mph. I had the same aero drag at 70 mph that I used to have at 45 mph.

It acted as a horsepower boost at highway speeds. It had almost no effect on my 0 to 60 mph time, but had a huge effect on my 60 to 100 mph time.

The car was much quieter. Wind noise was greatly reduced, especially with the deletion of the external driver’s side mirror located about 2 feet from my ear. Road noise was also greatly reduced, much of it being generated by turbulent airflow under the car that was eliminated by the smooth underbelly that I installed.

With most cars, air drag starts to become noticeable around 70 mph (60 mph with trucks), especially noticeable when coasting on downhills when it is slow to gain speed at highway speeds. On the Aerocivic, the air drag didn’t become noticeable until around 100 mph and it would gain speed quickly when coasting on downhills at highway speed. It took very little effort from the engine to maintain highway speeds.

The car stayed much cleaner. The dirtiest parts of a car’s surface are those subject to turbulent air flow, such as the flat surfaces on the rear of the car and around the wheel well openings. With a full boat tail and wheel well covers, the car stayed much cleaner. It’s rounded front also didn’t collect the load of dead bugs that the flat front bumper and grill do on normal cars. An occasional big beetle would ping the windshield, but most insects were carried up and over the top of the car in the smooth airflow without splatting.

It was much less hazardous to wildlife. Birds flying low right in front of the car in what used to be the “kill zone” were now carried up and over by the smooth airflow. With deer, the rounded and sloped front of the car acted as a wedge to lift them up and over the top of the car, causing little damage to the vehicle (but would have been hazardous to anybody tailgating me). I handled two deer in this manner compared to the extensive front damage incurred in two previous deer collisions before I aeromodded the car.

It created much less of a dust plume when driving down a dirt road, less of a water mist trail when driving on a wet road, and less of a turbulent snow trail when driving through snow.

Normal cars use the difference between the high pressure at the front of the car and the low pressure at the rear to move ventilation air through the car’s cabin when the vent fan isn’t running. As my car’s Cd dropped, this pressure differential dropped, this airflow dropped almost to zero, so I had to run my vent fan on low to get any air movement through the cabin.