They're one or the other, "how you read the tape measure" does not determine the
actual backspacing. Lay a straight edge across the outer edge of the rim (flat against the rim, not the tire) and measure straight down to the wheel mounting surface, the measurement at the edge of the straight edge is your backspacing.
If they're brand new and 15x6, they're probably these, which have a 3.75" backspace (+6 offset).
Wheel Vintiques 69-5612334 Wheel Vintiques 69 Series Chrysler Police Black Wheels | Summit Racing
225/60/15 would still be the largest you could go.
