2nd pic was taken in Australia, sorry.
TL;DR: 46.7MPG tank average, 473 miles on 10 gallons of gas. That’s with some gas spent screwing around too.
Got it all done up, tested it out. Worked great, although I didn’t have daylight or time to observe the airflow down the sides. The flow down the rear glass was better, with the separation bubble minimized but still present. I ended up staying awake very late getting everything bolted/taped up, and noticed a rock (from where the car was parked) which had strange slime on it that looked familiar. With it all attached to the car, I went and checked over it all to inspect any mechanical issues before putting on my skid plate/front diffuser.
The axle had torn its boot and shot grease everywhere! The rock was a red herring. I stayed up all night thanks to that. Good fun. Pays to have a parts car.
Anyhow, the car is doing really good! I had spent something like 50-60km doing speed runs back and forth to test aero, and left town shot about 2/8 a tank. With all my current modifications I have gone 762km on 38 litres of gas. In Americanese, that’s 473 miles on 10 gallons. I still had more in the tank, but decided to fill up. So safe to say I can hit 800km with a bit of effort.
Stability is good too. I think the spill plates help some. I was behind a big truck wavering all over the road in crosswinds while I didn’t have to steer much at all. Great area to put stuff on while the car’s parked. Picnic table?
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freebeard, I have considered both the Sierra biplane spoiler and raising the lid. This car is supposed to be my daily driver while I build up custom aero-mobiles from my many 90’s civics. So basically I want this all to be removable. That almost rules out the biplane spoiler as it attaches to the C-pillar and needs careful reinforcement. Raising the deck is easier, removable, and in the cards. Visibility of the third brake light will be tricky.