Summary:

Road vehicles such as passenger cars, trucks and SUVs are categorized as bluff bodies.
As a vehicle moves forward, the motion of the air around it produces pressure gradients,
which vary across the entire body surface. This in turn helps to develop a turbulent wake
region behind it, which works against the vehicle’s forward motion. As a result, the
engine/motor has to work harder, thus increasing fuel consumption.

Model files:

WishboneVG-Spacer.stl  allows to the proper spacing between the VGs. If the VGs are too close together then the benefit of the VGs are removed and cause drag.

Wishbone.stl - print in UV resistance and high temperature filament like ASA.

PrusaSlicer's Parameters

1. Overhang threshold: 60 degree
2. Generate support material: OFF
3. Fill density: 100%
4. Extra perimeters if needed: On
5. Vertical shells Perimeters: 4
6. Layer Height: 0.30 mm DRAFT
7. From 21 mm height, change the layer height to 0.10 mm to make post-processing easier.

Post-Processing:

1. Sand top model with 800 to 1500 grit. Wet sanding is the best way.
2. Use ceramic coating on top of model.

Installation:

These instruction are for Tesla Model Y. For other vehicles, it be best to print out 3 VGs and follow steps 1 to 4. Add more VGs when needed. Use the tuft method (https://www.amateuraerodynamics.com/2022/02/tuft-testing-how-to-manual.html) to find the best location for the VGs.

1. Find the middle of the rear window.
2. Using E6000 glue, cover the bottom of the VG.
3. Place the VG with the twin points  on top, ½ inch below the top of the window, taped the VG in place and let it set for 30 minutes.
4. Tape the spacer to one side of the first VG, repeat step 2 & 3. Continue until the outside edge is reached.
5. Repeat step 4 on the other side of the middle VG. 

Benefits 

When properly installed, the following things will be noticed

1. Able to maintain cruising speed with better mileage.
2. Less noise. 
3. Less back-end wobbling since there is no vacuum bubble lifting the back-end up.
4. Rear window clearer.

The vehicle will coast longer when accelerate pedal is release, so pay attention to the vehicle's speed. 

References:

See page 17 of reference PDF for information on  Boundary Layer and Separation of Flow

[https://dspace.calstate.edu/bitstream/handle/10211.9/1780/Sevilla%20-%20Thesis%20Final.pdf?sequence=5](https://dspace.calstate.edu/bitstream/handle/10211.9/1780/Sevilla%20-%20Thesis%20Final.pdf?sequence=5)

The YouTube video gives a visual description on how VGs works.