How do I check the accuracy of a speedometer?

FAQ Category
Equipment
FAQ Answer

A vehicle's speed can be made by measuring the time it takes to travel one mile. The best way to conduct the test is to set  cruise at one speed and measure the time it takes to travel between two green mile markers on interstate or other US/state highways.

Using the seconds on the left on the chart below, the corresponding mph is listed in the right column. 

Seconds Miles Per Hour
45.5 79
46 78
46.7 77
47 76
48 75
48.5 74
49 73
50 72
50.5 71
51.5 70
52 69
53 68
53.5 67
54.4 66
55 65
56 64
57 63
58 62
59 61
60 60
61 59
62 58
63 57
64 56
65.5 55
66.5 54
68 63
69 52

(Revised 07-2003)