If its the Gen three Auto in it 3.50 would be a good place to start. 1st gear with this trans is 3 to 1 . We put a petty garage (Arrington) Eaton V3 @8lbs and a true locking diff with 3.73's in the challenger. 1st ,2nd,3rd, are useless with the 20" F1 stockocrapo tires, so 2 more psi boost and 800 less pounds and some rubber that will hook watch out "Somebody gonna get hurt real bad" lol lol...