Rim-Dependent Section Width Estimator
Estimate a tire's section width on any rim width within its approved range. Uses the ETRTO 0.4 rule, with results in mm and inches.
Estimate width changes within approved ranges.
Rim-Dependent Section Width Estimator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Rim-Dependent Section Width Estimator Works?
The Rim-Dependent Section Width Estimator works by starting from the nominal width on the measuring rim and adding 40% of every millimeter of rim width change. The tool uses 4 steps.
Enter the tire size
Type the size, for example 225/45 R17, which sets a nominal 225 mm width.
Enter the measuring rim
Type the measuring rim, for example 7.5 in, or use the default for the size.
Enter the approved range
Type the minimum and maximum rim widths, for example 7.0 in to 8.5 in.
Read widths across the range
The estimator lists the section width on every 0.5 in step and marks rims outside the range.
Rim-Dependent Section Width Estimator Formula
The Rim-Dependent Section Width Estimator formula is SW(R) = SW_nom + 0.4 × (R − R_meas) × 25.4.
SW(R) = SW_nom + 0.4 × (R − R_meas) × 25.4ΔSW = 0.4 × 12.7 = 5.08 mm- SW_nom
- Nominal section width (mm)
- R
- Rim width (in)
- R_meas
- Measuring rim width (in)
Worked example
A 225/45 R17 tire with a 7.5 in measuring rim measures about 219.9 mm (8.66 in) on a 7.0 in rim and about 235.2 mm (9.26 in) on an 8.5 in rim. The full approved range spans 15.2 mm (0.60 in) of width.
Rim-Dependent Section Width Estimator Conversion Table
The Rim-Dependent Section Width Estimator conversion table shows the estimated width of a 225/45 R17 tire on 6 rim widths.
| Rim width | Section width (mm) | Section width (in) | Change vs nominal | Approved range 7.0–8.5 in |
|---|---|---|---|---|
| 6.5" (165 mm) | 214.8 mm | 8.46 in | -10.2 mm | Outside range |
| 7.0" (178 mm) | 219.9 mm | 8.66 in | -5.1 mm | Inside range |
| 7.5" (190 mm) | 225.0 mm | 8.86 in | +0.0 mm | Inside range |
| 8.0" (203 mm) | 230.1 mm | 9.06 in | +5.1 mm | Inside range |
| 8.5" (216 mm) | 235.2 mm | 9.26 in | +10.2 mm | Inside range |
| 9.0" (229 mm) | 240.2 mm | 9.46 in | +15.2 mm | Outside range |
What is Rim-Dependent Section Width Estimator?
A Rim-Dependent Section Width Estimator is a tool that predicts how wide a tire measures on a given rim width inside the tire's approved range. One tire size spans 10 to 20 mm (0.4 to 0.8 in) of real width across the approved range.
The estimate uses the ETRTO 0.4 rule. Real tires vary by construction, so the estimate works for planning and the tire maker's measurement settles exact values.
Clearance math that ignores rim width uses the nominal width and under-predicts the width of tires on wide wheels.
What are the uses of Rim-Dependent Section Width Estimator?
There are 4 main uses of the Rim-Dependent Section Width Estimator, including inner clearance math and tire size selection.
Inner clearance math
Use the real width on the planned rim in clearance calculations.
Tire size selection
Pick a size whose width on the rim suits the car.
Sidewall profile
Predict whether the sidewall sits straight or bulges.
Contact patch estimates
Estimate tread width changes on a wider rim.
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