2018 Challenger T/A 392.
I ran 275/40/20 front on 9.5" OEM rims, and 305/35/20 rear on an 11" MRR rim with 24 offset with Firestone indy 500 summer tires. This was my daily for 22,000 miles from April to Thanksgiving this year.
I had to trim a slight bit of the
inner well plastic at the back because it was curled some. Maybe a 1/4" No biggie.
No poke past the fender. No issues, no problems. No rotation and the tires wore evenly across. Rears wore 2x as fast as the fronts. When I took them off last month, the fronts had prob 7/32s Rears were at 4/32s and are a few good burnouts from needing replacement. Car had no weird handling issues and seemed to turn easier out of the corners. Def
The 315 tires are wider enough that Im told Id have to put in a 3mm spacer and possibly heat gun to form the plastic wheel well back in some spots to keep the tire from rubbing. Theyd fit though. 305s fit these rims but they have room to easily fit a 315. More room on a charger and spacer isnt necessary. Cant speak for what 21 offset will give you.
Id talk to one of the wheel fitters here on the boards who have done a lot of custom stuff for guys and see what they suggest you should go with.