The Bugatti Veyron Super Sport is the fastest car money can buy, after disabling the top speed limiter. Those last ten miles per hour are off limits, because Bugatti hasn't found tires that are safe at 257.8 MPH. Because of this the Guinness Book of World Records has decided that the Super Sport doesn't qualify as a pure production car. The software was beyond manufacturer specification, so it can't hold the title of fastest production car.
Basically, the committee is on Hennessey's side.
[via The Sunday Times]