[michiganpat]

offset measures from the the wheels centerline.

OEM charger 20x8 wheels are +24mm offset, meaning for an 8" wide wheel (measured at the bead seat, actual width is about 9" outside to outside), the face the wheel mounts on is pushed 24mm to the outside.

a 20x9 wheel with +20mm offset will have the face of the wheel pushed 20mm to the outside. that means that of the 25.4mm of increased wheel width, 8.7mm (~11/32") is pushed to the inside ((25.4/2)-4mm offset difference) and 16.7mm (~21/32") is pushed to the outside.

you should be fine...might get kinda tight if you couple them with much wider (say 275/40R20 tires, which are the same height, but 30mm (~1 3/16") wider than the OEM 245/45R20's...