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I think the improvement it's pretty small, mostly because the rear roofline & spoiler are already shaped decently for low drag.
If I recall, when I tested the kammback on the Firefly, it was good for something like a 4% MPG gain at my typical testing speed of 80 km/h. But the rear of the Firefly is much less optimized from the factory than the Mirage back end.
When I tested a smaller kamm as just one item in a group of mods on a Mirage I was loaned 5 years ago, the whole group was good for an improvement of 4.5 % @ 90 km/h / 56 mph.
Edit: another Mirage Forum member made and tested one and couldn't detect any difference. Not sure of testing methodology, though.
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