Yep, you're right, Johnny. Since Aaron's Dart has 205-60-R15 in front and 235-60-R15 in back, there isn't a single tire size we can use for the spare that is equivalent to both. However, I was hoping to find one that is close enough to their two sizes that also would fit into the tire well (and allow the tire cover to lie flat). That's not too much to ask for, is it?
The closest I found to being what I need is a temporary donut spare off a 90-something Jeep Cherokee. Its diameter was a close average of the two tire sizes, and it is skinny enough to drop below the lip of the tire well. Now I need to find one and try it.
Thanks for the help,
Jerry