Here is the info from that tire calculator site....
Speedometer Difference
Speedo Reading Actual Speed
20 mph (32.19 km/h) 21.02 mph (33.83 km/h)
25 mph (40.23 km/h) 26.27 mph (42.28 km/h)
30 mph (48.28 km/h) 31.53 mph (50.74 km/h)
35 mph (56.33 km/h) 36.78 mph (59.2 km/h)
40 mph (64.37 km/h) 42.04 mph (67.65 km/h)
45 mph (72.42 km/h) 47.29 mph (76.11 km/h)
50 mph (80.47 km/h) 52.55 mph (84.57 km/h)
55 mph (88.51 km/h) 57.8 mph (93.02 km/h)
60 mph (96.56 km/h) 63.06 mph (101.48 km/h)
65 mph (104.61 km/h) 68.31 mph (109.94 km/h)
