Need Advice: 16" wheels in 86 Cabby

The rolling radius of the tire is what determines whether the speedo is affected. I have 185-60x14 tires on my '86 Cabrio which are about the same rolling radius as the 175-70x13's that came on the car. With 16's, you'd need really low profile tires like 35 or 40 aspect ratio, and fairly narrow tires, like 185 or 195. These tires and wheels, though, would cause the car to ride badly and would tend to shake it apart.

If you really want to do this, probably tire rack could tell you what offset you need.