i'm pretty sure an R8
considering the fact that the r8 is a coupe compared to the rs4 which is a sedan, the r8 is goin to be faster just due to the fact that it weighs a lot less. They do however both have the same 4.2 liter v8, but the r8 is .2 seconds faster from 0-60.
The R8 is faster, especially around corners, however it is not that much lighter, only slightly... B7 RS4=3703#, R8=3439# or approx. 250# difference, in comparison, the B5 RS4=3571#, only 150# more than an R8...
http://www.albeedigital.com/supercoupe/articles/0-60times.html ^^^^^^this is the "How fast is a car stock?" bible. pretty much contains everything. its from the big name magazines, road and track, motor trend, and car and driver
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