I cant believe how everybody here points blame at oem spoilers for adding drag.
You people act like if its an oem spoiler, no matter what car or what kind of spoiler, it must just be another tacky add-on for looks.
Sure there are some cars out there where that may be true. But certainly not all. You guys really think any reputable car company just throws on any spoiler that the designers come up with? Cmon guys.
Some cars even have stated facts for optional "aero packages". This info can be found in the cars' brochere, such cars like some Saabs, lexus, and infinity. Heres a quote from infinity in fact:
Comprehensive airflow management, with underbody sculpting, for low 0.29 coefficient of drag (Cd) and zero-lift front aerodynamics (0.28 Cd and zero-lift front and rear with optional Aero Package)
The aero package includes a rear spoiler shown here:
Heres a quote from Honda about the spoiler on the civic:
The Civic Hybrid and Civic Coupe models use a lip spoiler on the trunk that cleans up the airflow behind the vehicle and it creates downforce at high speeds.
The fact is many oem spoilers actually
reduce drag. On many 3 box sedans small lip spoilers and low-rise spoilers help the air coming over the back window reattach and straighten out. On wagons and hatches like the yaris, a spoiler like the oem yaris one, prevents the air from following the downward curvature of the back glass and keeps its going nice and straight past the vehicle.