Compare the Ford Mustang FR 500 - [1999] vs Audi A6 RS6 Avant Plus 4.2L V8 Twin Turbo - [2004]. Find out which car is the fastest, most powerful, lightest or has the biggest engine in our top trumps style comparison page. Find out which car has the highest top speed or is the fastest accelerating, is it the Ford Mustang FR 500 or the Audi A6 RS6 Avant Plus 4.2L V8 Twin Turbo. We'll leave it up to you to decide which is your favourite.