[allanlambo]

The Speed limit was set by insurance company. The 911Turbo that ran 170mph was heavily modified. The Z06 that ran 176 on its last run was allowed to run to the 186mph limit because he had a roll bar, 4pt safety harness etc.

In the event you could do all the performance mods you wanted, but if your car could exceed the 168mph you had to have roll bar and harness's.

For example the ZR! that ran 159.5 in the mile, had the following mods:
90 ZR-1 #0120 Red/Saddle
Watson Ceramic Coated Headers
Corsa System
Random Tech Cats
Corey Henderson Calibration
4.10s
Corey Henderson Polished and Ported Plenum and IHs
Bill Boudreau's C4 Beam Plates
Bill Boudreau modified B&M Ripper Shifter (shortest throws on the planet!)
Centerforce Clutch (weights removed)
C5 Z06 Brakes up front
Stainless Steel Brake Lines
Goodyear F1-GSD3s

As for the Lp640, inmo, it would run right around 170mph in the mile. Also, again, remember this event was at an 1875ft elevation going into a headwind.