I write post-canon Spike as driving an Olds 88, because he loves his long, black "don't fuck with me" American cars.

Similarly, Buffy drives a sensible Toyota-type car, because she just wants a simple, reliable vehicle that can carry stuff and she has enough things to worry about beyond getting from place to place. Though if she has to go a long way, she leverages her resources for private jets - because, again, she has enough things to worry bout already.