Will winter tires fit in the trunk with the rear seats up?

I had picked up a set of 17 inch Pirelli tires mounted on steelies (all from Costco) and was able to fit them in the trunk only. Did not utilize the rear seats. So you should be alright. Just make sure your trunk is empty.
 
Barely. The will all standup back there, but it is a perfect fit. When we replaced the tires on our GT, I put the four new tires in the back to haul them to the shop to get them mounted.
 
I just re read your question. I've always folded the seats down. I tend to have a bunch of work related stuff in the back at all times. I'm no help.
 
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