20 Inch305/45-R18 tires are 2.13 inches (54 mm) larger in diameter than 245/45-R18 tires and the speedometer difference is 7.4%www.tacomaworld.com
Also has the car had lowering springs installed ?305/45-R20 tires are 2.13 inches (54 mm) larger in diameter than 245/45-R20 tires and the speedometer difference is 6.9%www.tacomaworld.com
The Speedometer would need to be reset by a Flash Update to the PCM
The computer only needs to be reset if the outside diameter of the tire has been changed. And even then if it is less than 10% the only thing affected would be mph. Example 245/45/20 has the same diameter as 275/40/20. 305/35/20 is 3/10 of an inch larger. No need for adjustment . Those wheels are 20x9 so you really don't want to go wider than a 275. I have not seen anyone making them in a wider size.Your Computer has to be Reset by a Dealer, Tireshop or depending on year of car there is hand held units that can do it.
How ever we can not effectively help you with knowing the Year and Model of Challenger and info I asked you.
This is bad advice.. you can force a 305 and maybe a 315 on a 9 inch wide wheel but the shoulder of the tire would not touch the pavement at the recommended 32 PSI. you could MAYBE make it work with 24-25 PSI but then the center of the tread will cup at speed....bad idea...Yes your Wheels will support up to 315 / 20 actually might have to use a heat gun and rubber hammer to gently massage the liner for those depends. A 305 will go on what you have. Make sure you install Rock Guard / Mud Flaps on the back unless you want to trash the rear quarter panels though.