auto cross

No reason they can't be street driven.

I ran Firm Feel 1.12" torsion bars in my Challenger and I put over 70k street miles on it, those are advertised at 270 lb/in, so stiffer than the Hotchkis E-body bars. Hell most of it was with absolutely abysmal KYB gas-adjust shocks. Switching to Bilstein RCD's made that car a completely different animal. I ran 275/40/17's on it, square. So maybe a little more sidewall than what's on that green Challenger but not much.

I have 1.12" Firm Feel torsion bars in my Duster, I've done over 30k street miles with it. The wheel rate on those is 300 lb/in, and honestly with the Hotchkis Fox shocks it rides amazing. I've actually considered bumping the wheel rate up a little, maybe if I can get something in the 320 lb/in range. And I run 275/35/18's on it in the front, with 295/40/18's out back. It's also a 16:1 manual steering car with +6.5° of caster. Sure, parallel parking is no fun but over 10 mph the steering is fine. Heavy, but fine. And above 45 mph it's amazing.

Currently my trip to work is 60 miles one way with over 3k feet of elevation change, I do that about once a week and some of the roads I drive would definitely be considered "back roads" whose surface isn't exactly smooth.