If you've ever tried to find info on MX6.com about using after-market side skirts from other vehicles on the mx-6, you no doubt ended up with a sense of confusion and frustration. That said, I was thinking about it, and think I've come up with a way to help make sense of the issue.
Since the aftermarket sideskirts are based on the distance between the front of the rear wheel well and the back of the front wheel well, it stands to reason that by comparing the 6's "skirt" length with other vehicles' skirt length, we can get at least a reasonable idea of which skirts would be workable on a six.
Please keep in mind that any skirt not manufactured for the mx-6 will need some work in order to make it fit. IMO, it would probably be easier to trim down a skirt that is longer than to try and lengthen a skirt that is shorter...but I'll leave that up to you. The purpose of this thread is merely to get some hard numbers.
So, everyone get out your tape measures, walk out to your parking lot and measure some cars. We should at least get measurements for the 2000-up celica, the two prelude styles, and the two recent 240sx styles, along with anything else you guys want to measure. Who knows? we might be able to solve the great mystery of customizing side skirts.

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MX-6 LS
Skirt Length = 73" (very close if not exact)
Toyota Celica (2000 - up)
Skirt Length = 73" (very close if not exact)