[brucepick]

I appreciate your effort to inform us here. But once again, I've discovered how little I know on this topic.

Are you saying that even the best optimized spoiler can't achieve much improvement in drag? Do you think that also holds true for cars like the Civic coupes that have a fairly gentle rear glass slope?

Beyond that concern, I have to admit two other confusions, right from the start.

I'm guessing that backlight angle is compared with a horizontal line continuing rearward from top of rear glass? So a 10º angle is a very gentle downslope, and a 0º angle would be a rear glass that is horizontal?

And pretty much the same question for the % aft-body length item. I'm guessing that refers to the dimension from base of rear glass (or from top glass edge?? to the rear bumper edge, straight back to a point even with the rear bumper. Am I close? That dimension as a percentage of what??

I'm sorry. A little knowledge is a dangerous thing. I thought I was going to build a rear spoiler, but maybe I'd be better off addressing other possibilities.