Moonmonkey - Boattails have been added to sedans, for example wagonman76's 4-door Celebrity iirc.
But before you go there try a few other mods, like rear wheel skirts and a full bellypan, both of which improve the airflow on the 'tail if and when you decide to add it.
You may also try a partial roof extension without sacrificing functionality, as in
Groar's Uglyback.
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e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread