The wheelsize/tire height is the ultimate factor. It will change engine revs so radically you won't know what happened. Decide how you think your are going to want to drive the car 1st and work out the gear ratio relative to the desired tire height. Changing tire aspect ratio- profile height- can even keep gear ratio the same if you go larger/smaller on wheel size. And finally don't forget to have a decent speedo shop calibrate your speedometer once you're all set up. Hopefully they can regear it as needed. Cops don't buy that crap about "uh, the tall wheels and tires musta threw off my speedo."