Figuring out the effect of tire changes on MPH / KmPH sppedo and odo is EZ.
Lets use 165/70 R 13 as an example.
165mm is the tire size. Aspect ratio is the height of the tire divided by width - in this case 70%.
So, tire height is 165 * .70 or 115.5mm.
13" is 330.2mm.
Diameter of the tire is thus:
115.5 PLUS 115.5 [both sides of tire] PLUS diameter of wheel [330.2] -
yields a total diameter of 561.2 mm or 22.09"
Multiply that times Pi and you get a rolling circumference of :
22.09 * 3.14159 = 69.41" or 5.78 feet.
Metric is 1763.06mm or 1.763m.
This tire will rotate 913 times in a mile, or 567.2 times per Km.
Trade that up to a 195/60 tire and do the math -
195*.6=117 time 2 plus 330.2 = 564.2mm [22.21"]
This tire rolls 908 times per mile [563 / Km] making the speedo off by only 1/2 of one pecent, not enough to make a difference.
change that new tire to 185/60 and you get that tire rotating 928 times in a mile, or a 1.5% differnce, reading higher speeds and more mileage driven than actual. Really not that dramatic.
Big differences happen with truck/SUV and ballon tires - not Fiats!
