OE wheel and tire specs??

The car currently has 18x9 245/40R18 rear and 18x8 245/40R18 on front. I have no spare.
I want to fit a new staggered set of wheels and tires on the car and have a spare that fits in the trunk well.
I also want skinnys on the front, but with enough meat on the ground to still corner well enough on the road.
I want Cragar 08/61 or 61C SS wheels in 15"
The rimstires.com fitment calculator tells me I should run 15x8 245/60R15 rear and 15x7 215/65R15 front.
I want to match the OE diameter (25.4") as best as I can to keep the speedometer accurate so I am thinking about 15x8 245/55R15 rear and 15x6 195/70R15 instead.
I figured I would just get a steel wheel for the spare, original 14" or preferably a 15" with a 205/70R15. I THINK that would fit in the trunk well ok.
Do you more experienced guys have any input or feedback you could provide.?