Speedo Accuracy and Wheels

Originally posted by Dennis
You can determine the amount that your speedo is off by finding the circumference of your current tire/wheel combo and comparing it to the circumference of the factory tire specs.

Note below the 78 aspect ratio, stock tires in the 70's were 165SR15 which can be translated to an 80 but 78 is more accurate. A larger overall diameter will slow the speedo down and a smaller diameter will speed it up.

135/78R15 = 23.29
145/78R15 = 23.91
155/78R15 = 24.52
165/78R15 = 25.13
195/50R15 = 22.68
205/50R16 = 24.07
205/40R17 = 23.46
215/40R17 = 23.77
225/35R17 = 23.20
235/35R18 = 24.48