[nexus_2006]

Hello everyone. I have a mk6 GTI, and my goal is to improve high-speed MPG. I get fairly good mileage below about 65 mph, but above that speed the efficiency drops off dramatically.

I've noticed that the Cd of the mk6 is often given as 0.33-0.35 depending on source, but the newer mk7's Cd is 0.27. I often only get 27-29 MPG at 70mph, but I know people with mk7 GTIs getting in the mid to upper 30s at that speed or higher (both cars have similar powertrain). The mk6 and mk7 look fairly similar, I believe I should be able to improve my Cd, although I know I probably won't reach the 0.27 the newer design has without killing the aesthetics.

I've been watching some Julian Edgar videos and I think I could reduce the vortex drag on the rear end with some sharper edges to induce flow separation - think things like rear glass side fins, and a ridge on the lower bumper where it starts to curve inward. I also plan to clean up the underbody a bit - there is an OEM underbelly pan I can get for $80 that covers about the front third of the car, OEM rear wheel suspension covers for about $40, and I'd also like to fabricate a custom underbelly for the rear axle back to the bumper (there is an OEM part for that too, but its been out of stock for years).

Does anyone have advice or opinions on these specific mods (rear window side fins, rear bumper separation edge, underbody cleanup in the front and rear)? I'm brand new to thinking about aero at all, but appreciate any advice, even if that advice is "don't bother" or "focus on something else".

I plan to make mockups with coroplast and masking tape, and do some wool tuft tests. I would make the upper body pieces with clear acrylic sheets molded with heat and cut with a router. I'm not sure what I would make the underbody pieces out of yet, or really how to test them.

Thanks!