[demick]

In my 914, I could fit 1 tire in the front trunk, 1 tire in the rear trunk, and 2 tires in the passenger seat.

When I got the Boxster, I played with the same idea. I don't remember if I could fit a tire in the front trunk (with the spare removed), but I don't think so. I definitely could not fit a tire in the rear trunk. And it's not realistic to fit more than 2 tires in the passenger seat. Bottom line is: you can fit some, but not all 4 tires.

Get a tire trailer.