[Phase]

Originally Posted by aerohead

1) From the side, any tail extension would fall away rearwards, beginning at 16-degrees, and slowly increasing in curvature, up to a maximum of 22-degrees.
2) If you went out to 12-inches, you'd be at 16.5-degrees.
3) At 24-inches, 18-degrees.
4) ' the-blueprints.com' has a dimensionally-correct image of your car that I've analyzed. Print one of those.
5) Scan the blueprint and place it under the Template, then trace off the elongation contour.
6) Print that, and take it to KINKO's or some place where you can enlarge it to at least 11-inches by 17-inches.
7) This will be your 'blueprint' from which to make scale measurements from.
8) You should buy a stainless steel, 18-inch millimeter rule from an art supply or drafting supply store.
9) And you need a pocket calculator.
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Whatever you build, it needs to begin, where the existing spoiler leaves off.
Just like Mercedes-Benz's IAA and EQXX
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For your 90-mph driving, I'd consider getting a receiver hitch installed, and wait for HARBOR FREIGHT TOOLS to put their lightweight aluminum rear cargo platform on sale. This would make a great foundation on which to construct an easily-removable tail section, which can be firmly attached to the Ioniq without mutilating it.
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With 24-inches, your aft-body percentage would rise to 56.7% of overall length.
Car's with this aft-body average around Cd 0.18.
The box-cavity you'd like to build would never help you significantly. And attachment options would be limited.
Sam Sharp bought Three Wheeler's Honda Insight-1 boat tail, and I think he's seeing 80-mpg the way he drives.
It's something to think about.
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As to the rear wiper, it should be submerged within the boundary-layer and if anything, just behave as a tripping device. Removing it might do 'zero' as far as mpg.

I’m
Saying the Ioniq has no rear wiper. The air flow is firmly attached.

And 2 feet is the range of extension im looking at actually. That would make the car the same length as the sonata.