225/40R18 vs 225/45R18: Calculating the Sidewall Difference

Nominal geometry for these tire sizes

The first size is the reference. Dimensions are nominal calculations, not measured rolling data or vehicle fitment approval.

Calculated dimensions are nominal, not measured rolling dimensions or vehicle fitment approval.

Identify the dimension that actually changes

Both markings specify a nominal width of 225 mm, shown in inches as 8.86. Both specify an 18-inch wheel diameter, also listed as 457.2 mm. Those fixed values make it easier to isolate what the aspect ratio does. The middle part of the code expresses sidewall height as a proportion of the width. It is not an independent length measurement.

The increase in the ratio is applied to the section width, rather than added to a sidewall as a millimeter amount. That distinction matters even when the markings look almost identical. Read the calculated sidewall entries to see the outcome. Comparing only the changed characters in the code would miss how the ratio interacts with the width shared by these sizes.

Read the worked sidewall example in millimeters and inches

For 225/40R18, the displayed sidewall height is 90 mm, or 3.54 inches. For 225/45R18, it is 101.3 mm, or 3.99 inches. The latter is taller relative to the reference. These entries demonstrate why the aspect-ratio change cannot be read as a matching change in millimeters. The code describes a proportion; the table supplies the resulting nominal lengths.

Use the displayed values to understand the relationship, but do not treat rounded entries as exact intermediate inputs. The table presents dimensions at a readable precision. Subtracting rounded sidewalls and then doubling that result can introduce a presentation discrepancy. The overall diameter entries provide the appropriate displayed totals, so there is no need to create a new result from rounded figures.

Account for the sidewall above and below the wheel

The reference diameter is 637.2 mm, displayed as 25.09 inches. The comparison diameter is 659.7 mm, displayed as 25.97 inches. Overall diameter includes the wheel and the sidewalls on opposite sides. A change in an individual sidewall therefore affects more than the upper edge of the tire. This explains the difference between a sidewall comparison and a whole-assembly height comparison.

Because the wheel diameter stays fixed, the taller assembly in this example comes from the sidewall change. It does not come from a larger wheel opening. Keep that explanation tied to this pair, where width is also unchanged. A different comparison could change width, wheel diameter and aspect ratio together, making a glance at the middle part of the code insufficient.

Follow the circumference change in the same direction

The 225/40R18 reference has a circumference of 2001.8 mm, or 78.81 inches. The 225/45R18 comparison has a circumference of 2072.5 mm, or 81.59 inches. Moving from the first row to the next therefore increases the nominal distance around the tire. This is a geometric relationship, not evidence of a specific distance recorded on a vehicle.

Revolutions per mile move the other way: the table shows 803.9 for the reference and 776.5 for the comparison. A larger nominal circle needs fewer turns over that distance. The values are not measured rolling data under load. Do not turn them into a promised speedometer correction or an assumed road speed; the comparison here establishes the dimensional direction without testing an instrument.

Separate the dimensional result from an installation decision

Similar-looking sizes and a shared wheel diameter do not amount to vehicle fitment approval. Check the vehicle instructions, exact tire specifications, approved rim widths, load and speed ratings, wheel details and clearance before treating a candidate as usable. The geometry does not establish the space available on the vehicle, and it cannot resolve a missing requirement in the documentation.

When discussing the change with a qualified installer, bring both full size markings and keep the reference identified. Record sidewall and total diameter as separate fields so the reason for the change remains visible. No pressure, load capacity, comfort benefit or fuel-use outcome is established by this calculation. A taller nominal sidewall is a dimension to investigate, not a promise about the driving experience.

Frequently asked questions

Are aspect-ratio points the same as millimeters?

They are different kinds of quantities. The ratio is applied to section width; the resulting sidewall height is then expressed as a length.

Why does the overall diameter change with the same wheel?

The tire contributes sidewall height on opposite sides of the wheel. Increasing that height increases the complete nominal diameter even with an unchanged wheel opening.

Which direction does the comparison use?

225/40R18 is the reference and 225/45R18 is the candidate. The candidate is taller and has a greater nominal circumference in the table.

Should I calculate an exact difference from the displayed sidewalls?

Use the displayed entries for interpretation. Further arithmetic on rounded values can lose precision, so it should not replace the original calculation or its displayed totals.

Continue your comparison

• 225/60R18 vs 225/65R17: What Changes Beyond the Wheel
• 205/55R16 vs 205/60R16: Sidewall and Diameter Compared
• Compare your own sizes in the tire size calculator