Originally Posted by Nibbo
Itís quite simple really.
Given the following:
PI is 3.142857143
To convert inches to mm you multiply them by 254.
Therefore the following aspect ratios will produce the following conferences.
A/r 65 = ((15 * 254) + 65) * 3.142857143 = 12178.57143mm
A/r 60 = ((15 * 254) + 60) * 3.142857143 = 12162.85714mm
This means that the tyre with the larger aspect ratio will travel an extra 15.71428572mm per rotation.
As a percentage this is about 0.129198966% which means that the 1,780 miles your odometer registered it could have been as much as an extra 2.299741602 miles (roughly)!
You may wish to note also that if your speedometer is registering 50mph you are probably nearer 51mph so watch those speed cameras.
It's not as simple as you thought
The section height of a 195/65 tyre is 195mm * 65%
= 126.75mm. You need to double
that and add to the rim diameter in mm to get the rolling diameter.