While the specs for each car are essentially the same, the difference the driver, set-up, tire pressure etc.etc , can make, discounting any change in track conditions: For Single Car runs at Daytona testing;
NASCAR.COM - Daytona test speeds - Jan 16, 2008
From the Hendrick Team, where all four cars are essentially the same:
Earnhardt: 185.820
Johnson: 184.782
Mears: 184.574
Gordon: 183.809
Johnson & Gordon have the same shop . During the week, each Team wears identical Hendrick shirts, not their Team sponsor shirts. On a 2.5 mile track, the speed/time difference is significant regarding track position
Interesting to note that the fastest time was 185.820 ( Earnhardt ) and the slowest was 176.856 for 56 cars tested
A common complaint, unfair competition, in the past is the rules favored one make over another regarding the aerodynamics, In the pros and cons of specs, seems to now be a non-issue