It cost me $100 to get the speedo recalibrated on my '93 after going from 14s to 15s. Your speed differential will vary. In my case, it was about 3 mph difference in the 30-40 mph range and got up to about 7mph when I got to around 75. However, I have 2 Capris and could never remember in the moment which one I was in, so I bit the bullet and had the recalibration done.
That site that Matt posted will show you the variance for each tire size/width/height.