drag coeffient does matter at normal speeds, hints why the honda insight that will probally never see speeds over 115mph has the lowest DC of any production car ever produced. For the RSX, when the factory performance kit from acura (lowering springs, front air dam, body kit, and spoiler) the car tops out 2-3mph faster (reported in car and driver I think when the kit was released) Also why I get 1 mpg better when I tuck my mirrors in on long country-road drives.