Tire Diameter Calculator

275/70R18 is the closest indexed production size to 33.0" on 18" wheels in the current reverse search. For your 33.0" target on 18" wheels, the closest indexed size is 275/70R18 at about 33.16" overall. Overall height is calculated from the size code (section width, aspect ratio, and wheel diameter), then ranked by absolute difference from your target. Published model catalogs can still list a slightly different diameter for a specific product. Diameter matching is not a fitment confirmation — confirm clearance, load rating, and vehicle requirements separately before purchase.

Nominal overall diameter is calculated from the tire-size geometry: sidewall height = (section width × aspect ratio ÷ 100), then overall diameter = wheel diameter + 2 × sidewall height (with millimetres converted at 25.4 mm per inch). Example path for a metric size such as 275/70R18: sidewall ≈ 7.58", overall ≈ 33.16". Circumference is π × overall diameter, and revolutions per mile equal 63,360 ÷ circumference in inches. TireReference uses these nominal calculations for reverse-search ranking. They are not the same as a tape measurement on a loaded vehicle, and they are not substituted with individual product-model published diameters for generic size ranking.

No. Wheel diameter (the R-number) is only the rim bead-seat diameter; overall tire diameter includes both sidewalls and the tread height. An 18-inch wheel does not mean an 18-inch tire. Your current closest match 275/70R18 calculates to about 33.16" overall — taller than the wheel alone by two sidewalls. Use wheel diameter for mount compatibility and overall diameter for height, speedometer, gearing, and clearance estimates.

Calculated diameter is a nominal size-code value for reverse-search comparison; a driveway tape measure reflects construction, inflation, load, remaining tread depth, and measuring-rim width. Those real-world factors can make a mounted reading higher or lower than the nominal figure without meaning the size code is wrong. Compare measured height to the calculator result as a screening check, not as proof of catalog error. Confirm critical fitment with your specific wheel, pressure, and vehicle setup.

Speedometers assume a fixed rolling circumference. When overall diameter increases, each revolution covers more ground, so true speed is typically higher than indicated at the same wheel RPM; the reverse happens when diameter decreases. Against your 33.0" target, 275/70R18 at ~33.16" changes circumference (and therefore indicated-vs-true speed) in proportion to that diameter difference. Wear, load, temperature, and OEM calibration can still shift real cluster behaviour. This tool does not recalibrate your instrument cluster.

Choose the wheel diameter you will actually mount, then search for metric (or flotation) sizes whose calculated overall diameter lands near your 33.0" target. Overall height equals wheel diameter plus two sidewalls, so the same overall target requires different aspect ratios and widths on different rim diameters. Your search is currently filtered to 18" wheels — change the wheel selector if the rim size is still undecided. Wheel width, offset, bolt pattern, and brake clearance remain separate from overall diameter.