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Tire diameter versus wheel diameter
The wheel fits inside the tire. Its diameter is only one part of the total.
Reviewed Oct 27, 2026
Two circles with different meanings
Rim diameter identifies the nominal diameter at which the tire mounts to the wheel. Overall tire diameter reaches from one outer edge of the tire to the opposite outer edge. In TireSpan’s nominal model, the outer circle is the rim circle plus two sidewall heights. Neither circle is a drawing of a brake disc or the outside edge of a rim flange.
An 18-inch rim can have a smaller tire around it
The default comparison is a useful example. 215/55R17 has a 431.8 mm rim diameter and 118.25 mm sidewalls, for a 668.3 mm overall diameter. 225/45R18 has a larger 457.2 mm rim diameter but shorter 101.25 mm sidewalls, for a smaller 659.7 mm overall diameter. The rim grows by 25.4 mm while the total diameter falls by 8.6 mm.
Read the changes separately
The new sidewall is 17 mm shorter on each side of the rim. Together those two reductions total 34 mm, more than offsetting the 25.4 mm increase in rim diameter. Looking only at the final rim-size number would miss both the sign and the scale of the overall change.
Diameter and radius answer different questions
A diameter change of −8.6 mm corresponds to a nominal radius change of −4.3 mm. Ground-aligned circles make this relation visible: their centers differ in height by half the diameter difference. That geometric result is not a measured ride-height prediction, because a loaded tire is not an undeformed circle.
Use the table alongside the drawing
Read rim diameter, sidewall height and overall diameter as three separate rows. The common scale makes the outline comparison honest even when the difference is small. If the picture looks almost unchanged, inspect the numbers instead of expecting an exaggerated drawing.