235/40R18 vs 245/40R18: Low Profile Width and Height Compared

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.

Translate the shared profile into separate sidewall heights

An aspect ratio of 40 means that sidewall height is calculated as that percentage of section width. For 235/40R18, the displayed sidewall height is 94 mm. For 245/40R18, it is 98 mm. The profile label remains the same, but the underlying dimension changes because the width used in the calculation is different.

This distinction matters when you compare drawings or measurements. The number after the slash is neither a millimeter height nor a description of wheel width. Treat it as a proportion tied to the first number, and read the completed sidewall calculation before deciding which assembly is taller.

Add the sidewalls to the common wheel diameter

The worked diameter formula adds twice the calculated sidewall height to the wheel diameter, with all terms expressed in matching units. Both rows use an 18-inch wheel diameter, shown as 457.2 mm. The complete calculated diameter is 645.2 mm for 235/40R18 and 653.2 mm for 245/40R18. The candidate is therefore taller despite the unchanged wheel and profile markings.

For a comparison in inches, the corresponding diameters are 25.4 and 25.72. The sidewalls appear as 3.7 inches and 3.86 inches. Those columns convert the same nominal dimensions; they do not describe an additional tire size or a measurement taken from a product.

Do not turn section width into a wheel-width instruction

Section width is shown as 235 mm or 245 mm, with inch equivalents of 9.25 and 9.65. These figures describe the tire's nominal section, not the wheel's bead-seat width. The shared R18 ending supplies wheel diameter only. It cannot fill in the missing wheel-width requirement or establish that an existing wheel is suitable for the candidate.

When checking an exact tire, look for its specified measuring rim and permitted rim range in the product documentation. Keep those details in their own place beside the calculated section width. Mixing the labels can make a tidy comparison table answer a question it was never designed to answer.

Follow the taller diameter around the tire

The circumference formula multiplies diameter by pi. It gives 2027 mm for the original and 2052.1 mm for the candidate, so the latter has a longer nominal distance around its circle. This leads to fewer geometric revolutions per mile: the entries are 794 and 784.2 respectively. Each candidate turn accounts for more nominal travel.

Keep the term geometric beside those rotation results. A loaded rolling tire is not the ideal circle used by the calculation, and this table supplies no measured rolling circumference. Product data collected under stated conditions should retain those conditions when you compare it with the values calculated from the markings.

Read the speed example with its starting assumption

Assume the instrument originally corresponded exactly to 235/40R18, with no other indication error. The speed formula multiplies the indicated reading by the candidate diameter divided by the original diameter. At an indicated 60 mph, the modeled change to 245/40R18 produces a calculated actual speed of 60.7 mph. Its larger circumference explains the upward direction.

The other illustrated readings are 30.4 mph at an indicated 30 mph and 75.9 mph at an indicated 75 mph. They extend the same dimensional relationship rather than adding evidence about a real car. A vehicle's current calibration and the exact tire's rolling behavior remain outside the example.

Use the comparison to organize further checking

The similar overall sizes shown here do not approve fitting either tire to a particular vehicle. Vehicle documentation and exact tire and wheel specifications must resolve compatibility, required service description and clearance. A shared low-profile marking cannot substitute for those checks, and the numerical table does not provide an inflation prescription or load-capacity result.

Keep the conclusion focused on what was calculated: 245/40R18 is nominally wider and taller than 235/40R18, with a longer circumference and fewer geometric turns per mile. Avoid attaching claims about comfort, fuel consumption or driving behavior to that conclusion. Those properties are not outputs of this dimensional model, even when the arithmetic looks straightforward.

Frequently asked questions

Does a 40 profile mean the sidewall measures 40 mm?

The profile is a percentage of section width. In these rows the displayed sidewall heights are 94 mm and 98 mm, rather than the profile number itself.

Is 9.65 inches the wheel width needed for 245/40R18?

That value is the candidate's converted nominal section width. The required or permitted wheel width must come from the exact tire's documentation.

Which value describes height across the entire assembly?

Overall diameter includes the wheel and both sidewalls. For the reference row it is 645.2 mm, whereas the sidewall value describes only a part of that height.

Why keep the manufacturer dimensions as a separate reference?

They relate to an actual product and its stated measuring conditions. The calculated figures here come from the size marking alone, so the sources and methods should remain clear.

Continue your comparison

• Compare your own sizes in the tire size calculator
• Understand tire measurements
• Compare geometric circumference