The test condition can have very much influence on the result. See today f1 race, what happend with Alonso.

Here are some result from a race at London airport:
A British supercar set an astonishing speed record in the heart of London by storming down an airport runway at 175.7mph on Saturday 15 July.

The Pounds317,500 Mercedes SLR McLaren supercar hit the top speed in a one mile speed challenge at London City Airport, a stone's throw from ExCel, the venue for the British International Motor Show which was cleared of aircraft for the event.

Vehicle Record Attempt Speed
Mercedes McLaren SLR 175.7
Ford GT 174.3
Porsche Turbo 162.6
Bentley Flying Spur 157.4
Corvette Z06 167.0
BMW M6 162.5
Alpina B5 155.4
Shelby Daytona Cobra 159.7
Holden Maloo 160.8

There the Z06 beats the turbo and the M6 easily. M6 and 997 tt are equal.

AM