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Size comparisons

225/45R17 vs 235/40R18 — the sidewall runs out first

Where the plus-size ladder stops balancing, and why the overshoot always appears at the low-profile end.

225/45R1724.97 in tall8.86 in wide235/40R1825.40 in tall9.25 in wide+0.43 in · +1.72%
Drawn to scale from the size codes. Solid outline is 225/45R17; dashed outline is 235/40R18. The bars below each tire are section width at the same scale.

235/40R18 is 0.43 in taller than 225/45R17 (1.72%) and 0.39 in wider. A speedometer calibrated for 225/45R17 would under-read by 1.72%.

Measured against each other

  • Overall diameter

    24.97 in25.40 in+0.43 in (+1.72%)

  • Section width

    8.86 in / 225 mm9.25 in / 235 mm+0.39 in (+4.44%)

  • Sidewall height

    3.99 in3.70 in-0.29 in

  • Circumference

    78.5 in79.8 in+1.3 in

  • Revolutions per mile

    807.6794.0-13.6

  • Ride height at the axle

    ——+0.21 in

  • Speedometer reading

    ——+1.72% under-read

Calibrated for 225/45R17, running 235/40R18

Dial reads 30
30.5 mph
Dial reads 50
50.9 mph
Dial reads 70
71.2 mph
Odometer counts 100
101.7 mi covered

Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.

Down at the 40-series end of the scale, there is not enough sidewall left to pay for another inch of wheel.

Look at what is available to spend

The sidewall on the 17-inch fitment is 3.99 in. On the 18-inch fitment it is 3.70 in. The step from 45-series to 40-series only released 0.29 in — and half an inch per side was needed to absorb the extra wheel radius.

The shortfall has nowhere to go except into the overall diameter, which climbs 0.43 in, or 1.72%.

Why the steps get smaller as the wheel gets bigger

An aspect-ratio step is a percentage of the section width, and five points of a 225 mm casing is 0.44 in while five points of a 235 mm casing is 0.46 in. Those figures barely move as you climb the ladder, but the inch of wheel you are trying to absorb never gets any smaller. Early in the ladder one aspect step plus one width step covers it comfortably. By the 40s it does not, and every further step overshoots.

Consequences, such as they are

Each wheel turns 794.0 times per mile instead of 807.6, and everything downstream shifts by that same small ratio. Thirteen and a half revolutions in a mile is the whole of the change, and the practical significance of 1.72% on a road car is close to zero.

Where it does register is on the wheel arch and on the wheel itself. 0.21 in more radius and 0.39 in more section width arrive together, and a 3.70 in sidewall gives the rim very little protection when it meets a pothole edge.

Both of these are factory sizes somewhere

Neither code is unusual. 225/45R17 and 235/40R18 are both high-volume original fitments across compact and mid-size cars, which means this pairing is often a comparison between two versions of a car rather than a modification anyone is contemplating. The 1.72% drift exists in the factory's own range and was evidently judged acceptable, since both sizes were homologated on the same platforms.

The version that works

One step down the ladder, 205/55R16 to 225/45R17 achieves the same exercise with a 0.09 in error rather than a 0.43 in one. The difference between the two pages is the whole argument for stopping plus-sizing before the sidewall gets short.

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