The difference is more like 5% (21 is 1 more than 20, ie 20 + 1/20 * 20, and 1/20 = 5%). Fortunately, the speedo will now read higher than reality, so far less likely to get photographed.The error in mine, having gone from a GTS4 to a Stagea (4.083) diff, is roughly 7%. But mine reads under the real speed - caution, Will Robinson!