Hey Pete. I don't know 240s, but maybe I can get you started with the math. You'll need to know the width and offset of the 15's already on the car. Start by subtracting the offset of the present wheels from the offset of the Cetus. Then add in half of the width difference, if there is any.

For example, suppose your present wheels are 6" wide with a 25mm offset. In this scenario, the resultant number is 18. That would be the width of spacer needed to keep the center of the new rim exactly where it is. So if you use a 20mm spacer, the centerline moves outboard by 2mm. Then subtract the widths, which would be 1/2" or 12.7mm--let's round up to 13. So the edge of the rim will end up 15mm closer to the fender lip (13 + 2). The inside of the rim will be 11mm closer to the inner wheel well or strut. Then there is the matter of tire selection, which won't matter if you stick with the same size.

--Wilson