Figuring actual tire diameter is another source of potential error. This is often difficult based on the nominal tire size, particularly given metric sizing. This Flotation/Metric chart compares similar-diameter tires in flotation sizing (e.g., 31x10.50R15) to metric sizing (265/75R16). The diameters shown are approximate and will vary by tire but should be helpful. Cross-section widths are not given--flotation-sized tires are generally available in wider sizes. If you have a metric tire not listed, you can calculate (using the example size above) its approximate diameter by multiplying the aspect ratio (75 percent) by the cross-section width (265) and doubling it. Convert that to inches by dividing it by 25.4, and add the diameter of the wheel(16).