The 5 being new is not known yet in tire shops. I did reserve for wheels/tires and replacement for Mazda 3 GT setup (17" + mags). The tires/wheels are 16" and less tall so speedo will almost be exactly on.

Question is are wheels compatible between 3 and 5 models.

so youre trying to find wheels to beat on in the winter? why not measure your bolt pattern and find some cheapos, or see if mazda offers steelies?