Suzuki Swift Boot Spoiler, what's the effect?

[Brit]

My Suzuki Swift is quite highly specced so it has a small roof spoiler at the back. What I don't know is whether it has a negative, a positive or a neutral effect on my fuel efficiency so to the more experienced members; which do you reckon it is? Would there be any benefit or losses if I sealed the gap between the top of the tailgate and the top of the spoiler? I don't really want to remove it as it would leave holes in the tailgate where it is bolted in place.

[MetroMPG]

Hi there -

If the spoiler sticks up higher than a line extended from the existing curvature of the roof, it probably increases drag.

The Swift GT in North America (same body style as yours) had much a very low profile spoiler (more of a roof extension, really). One of our members here claimed after doing "max velocity" downhill runs that it reduced drag vs. no spoiler at all.

But even the unadorned hatchbacks have an integrated "lip spoiler" above the glass (integrated in the window trim).

I'd like to see that spoiler in profile before making a call, but I suspect it's something I'd remove, personally. You can always put metal tape over the holes or put bolts back in, then reinstall it later if you want to (eg. when selling).

Welcome to the forum, by the way. I saw your intro post but haven't had a chance to say hello yet.

[freebeard]

A zero-lift drag-reducing device would be below the daylight you can see below the installed device. See 'The Template' or 'Swift/Metro Kammback'.

Maybe the NA part would use the same holes. I doubt you need rear downforce on a FWD vehicle.

[aerohead]

I'm guessing a drag increase,however,if the backlight angle is around 28-degrees,the Swift would have bi-stable flow,alternating between squareback flow and fastback flow,creating a pulsing wake which the spoiler might help attenuate.
You'll notice sharp edges on rear bumper fascias of modern cars.In an aero-accoustic wind tunnel the original bumper may have been creating this pulsing,which is fixed by the 'burst' structure,fixing the drag coefficient,and lowering wind noise.