[lunarhighway]

small changes can have a big effect on drag, that's both good and bad news.

the opel/vauxhall calibra for example hads a drag coeficcient of only 0.26, but that's only true for the 8 valve versions as 16v's had no undertray, more grill opening, and wider tires.

their cd is 0.29

that sugests that adding and undertray, narrow tires and a grillblock, might decerease your cd by 0.03

i think the bodywork of your car is pretty smooth, but as in most cars, the underside will be a desaster, so that's where the simples improvements can be made, and they're mostely out of sight so it won't affect the looks.

unless you're tires need replaceing i wouldn't loose any sleep over it though. you might want to look into wheel dams, they seem pretty universal on current cars, and while i still haven't figured out exactly how big their impact is, their addoption by nearly every carmaker suggests they're a quick aero fix that impoves FE measurably.

that's nice about older cars with fairly good aerodynamics... they don't even have these simple tweaks applied, so little additions might make them even better.