I'm sorry i'm not sure if you are buying one or considering removing the one you already have. I would say if its already on it would make a minor difference of you took it off so don't worry about it (if you do then try some A-B-A testing to see what kind of difference it made), but i wouldn't pay money to get one, if you want down force I would go with a belly pan (perhaps with a built in splitter?) which increases front end down force slightly so less understeer while improving overall Cd.
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.
it shows that a clean cut trailing edge is rather important to reduce drag this is why some cars have a small straight "spoiler" at the rear of the roofline.
on the other hand most real spoilers that generate downforce also generate drag...on a car that has enough power to send it into the air, you might want to invest some of that power to keep it on the road.
a while ago i came across an article that showed that ,at least over here, the yaris is mainly bought by elderly people ... not exacly sexy... you don't want the granny image stamped on a smooth compact car, that wants to be young and trendy yet affordable and practical... so how do you underline that you didn't design the car to be driven by people who do half the speedlimit all the time...get some "sporty options".
(btw if i where to get a new car right now i'd sure make a testdrive in the yaris... it looks great with or without the spoiler)
i don't mean to say it's impossible there's some advantages to that spoiler... just that untill i see figures that prove the spoiler actually improves FE there's a chance that it's more for image than anything else.
even a carmaker with a rather green image like toyota knows that a cool car still sells better then an efficient one