Gurney flap on new beetle?
 

How big should it be how high, and really what would be the best shape? I used to have a 1985 Toyota Supra I did had a great big one at the top of the hatch I almost think it might need to be that big

Gurney flap (wickerbill) is small, then you get spoilers and then wings. Each function differently.

The turbo New Beetle had an active spoiler built into the top of the hatch. A wickerbill would be appropriate on it's trailing edge. It's kind of a spoiler on the spoiler.

A wing creates downforce, while a spoiler is basically a vortex generator.

You might want to review this thread: ecomodder.com/forum/showthread.php/aeromods-2001-vw-beetle-19668.html

http://ecomodder.com/forum/attachmen...2&d=1242528909

Julian [banned] Edgar made a similar wing with transparent side plates, but he only has four other threads remaining so it must have been in some other thread. It might be in his Autospeed pages.

Minimal:
http://i184.photobucket.com/albums/x...e/DSCF4698.jpg
ecomodder.com/forum/showthread.php/aeromods-2001-vw-beetle-19668.html#post274074

Maximal:
http://i.imgur.com/QMKpl.jpg
ecomodder.com/forum/showthread.php/aeromods-2001-vw-beetle-19668-2.html#post324256

What happened with Julian?

He got crosswise with the mods. Ask aerohead, I forget the details.

edit:
If you can't get enough, he has a new newsletter. Here's a Youtube video from six months ago: My new free newsletter!

No evidence offered for what I suspect, but the roof spoiler of the RSi looks promising.

https://www.coolcarsinchina.com/2018...-rsi-in-china/
https://i0.wp.com/www.coolcarsinchin...g?w=1045&ssl=1

Something on the top, might even make something on the bottom effective.

Bottom Example:
https://www.carid.com/sf-pl-mpn/pid-...E&gclsrc=aw.ds
https://images.carid.com/t5i/items/a...e-oncar-01.jpg

It's after the high point of the roof, so I wonder if it is operating in disturbed airflow and therefore partially effective. Suspect more drag than downforce BUT it is behind the CG so there would be a stability benefit.

' Gurney flap '
 

Any drag reduction is going to come from a pressure rise over attached flow, decelerating rearwards on a streamlined diminishing cross-section.
A Gurney flap can't do that.
A truncated Kamm-back is best.
Take a look at roofline 'silhouettes' Volkswagen chose for their :
- Volkswagen 'Flow-Body', L1, XL1
- Porsche 911 GT1, 918, Taycan.
- Audi E-Tron GT
- Bugatti Veyron, Chiron
You just chop the roof off like a loaf of bread, wherever the length is acceptable.

EDAG Biwak Concept 2006
http://ecomodder.com/forum/member-fr...llpaper-03.jpg
https://ecomodder.com/forum/member-f...llpaper-02.jpg

I love that wagon conversion

The size and shape of the Gurney flap really depend on what you're aiming for—better downforce, reduced drag, or just aesthetics. A larger flap, like the one on your old Supra, can help, but for a New Beetle, something more subtle might work just as well. Testing different angles and heights could give the best results without adding too much turbulence.