OK, I found some equations for calculating the amount of horse power (at the wheel) that would be required to balance air resistance (drag) at certain speeds. Or you can think of it as whp lost due to air resistance.

For the Crossfire SRT-6 Coupe, which has a Drag Coefficient of 0.370 and a frontal area of 20.56 sq. ft. here's some numbers:

Speed (MPH) _____ Resistance _____ WHP Req'd
=====================================
30 ______________ 18.4 __________ 1.47
55 ______________ 61.8 __________ 9.06
65 ______________ 86.3 __________ 15.0
75 ______________ 114.9 _________ 23.0
100 _____________ 204.2 _________ 54.4
125 _____________ 319.0 _________ 106.3
150 _____________ 459.4 _________ 183.7
175 _____________ 625.3 _________ 291.8
200 _____________ 816.7 _________ 435.5

Now those numbers don't consider the roll resistance of tires, which from what I've read is significant. I haven't found out how to figure out those numbers. That said, it seems obvious that a stock (not speed limited) SRT-6 (with about 290 whp) is going to top out just shy of 175 mph just due to air resistance.