I don't know for sure offhand if the factory alloys are the same as the steel, but since the wheels are off you can check easy. Set a straight edge on the inside of the rim & measure (perpendicular) to the inner wheel mounting surface. Then put the straight edge on the outside & measure to WMS... Adding the two = rim width, 2nd number is your backspace. 1st measurement is how much 'tucks in' to the wheelwell, 2nd is how much 'sticks out'. Stock steel are 15x5.5 & iirc 1.75" b/s. hth
edit: you'll need a 15" straight edge so you're not measuring the tire width too...
Fuzz