You've got to change the aspect ratio when you change the tire width or the height of the tire will change, too. Check out
www.tirerack.com for plus sizing tips. They suggest a 225/50-16 as a good option. When you changed the original 205/55-16's to 215/55-16 you increased the width, but left the aspect ratio the same, which made the tires slightly larger (taller) which screwed up your speedo. The computer's trying to compensate, but can't.