FWIW, I had:
- 1-3/4 on car when I bought it and kept them when it was a 12-sec car... 450-ish HP
- switched to 1-7/8 when engine got upgraded to run into the low 11s... 550-ish HP
- switched to 2" when another upgrade got solidly into the mid 10s... 600 HP
- keeping the 2" w/ the new engine combo that's making 650+ HP
2021 Challenger 6.4L Scat Pack 1320
100% stock: 1.680, 11.894 at 113.75 (DA 175 ft)
weight reduction, wheels, tires, Hellcat air box: 1.661, 11.686 at 115.97 (DA 710 ft)
1973 Challenger 452 ci street/strip [2008]
pump gas, DOT radials: 1.454, 10.523 at 126.44 (DA 514 ft)
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