Tire Size and Comparison Calculator calculate tire diameter from code and compare speedometer differences for tire sizes.
The first number 255 is the tire width in millimeters.
The 2nd number 8.5 is used to calculate the sidewall height or "aspect ratio", by percentage. So, the sidewall height of a 255/8.5 R 20 tire is 60% of 255 = 192
The 3rd number 20 (generally preceded by an "R") is Rim Diameter in inches. This is the diameter of the rims bead seat base, not the diameter of the outer rim edge. The rim edge, or lip, extends past the tire bead seat to hold the bead and stop the tire slipping off the rim.
When we say a rim has a 20 inch diameter, we mean that the diameter of the bead seat base measures 20 inches. The outer rim edge diameter will actually measure more, depending on the design.
If you replace 320/60/18 with 235/60/18 for Odometer reading 1000 the actual distance traveled = 879 ~ (-12.1%)
(If your Odometer is accurate for original tires)
Edit and Copy Tire diagram above to compare or print.
|320/60 R 18 KPH||235/60 R 18 KPH|
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