Measuring Rim Width Comparator
Compare the measuring rim a tire maker used for published dimensions with your actual wheel width, and adjust the section width.
Compare manufacturer measurement conditions.
Measuring Rim Width Comparator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Measuring Rim Width Comparator Works?
The Measuring Rim Width Comparator works by comparing the rim width a tire maker used to measure published dimensions with the rim width the tire will run on. The tool uses 3 steps.
Enter the published data
Type the published section width and measuring rim, for example 245 mm on 8.5 in.
Enter the actual rim
Type the wheel width the tire will mount on, for example 9.5 in.
Read the adjustment
The comparator applies 40% of the rim width change and returns about 255 mm (10.04 in) of section width on the 9.5 in wheel.
Measuring Rim Width Comparator Formula
The Measuring Rim Width Comparator formula is SW_actual = SW_published + 0.4 × (Rim_actual − Rim_measuring) × 25.4.
ΔR = (R_act − R_meas) × 25.4ΔSW = 0.4 × ΔRSW_act = SW_pub + ΔSW- R_meas
- Measuring rim width in the data book (in)
- R_act
- Actual wheel width (in)
- 0.4
- ETRTO rule-of-thumb factor
Worked example
A 225/45 R17 tire measured at 225 mm on a 7.5 in rim gains 0.4 × 25.4 = 10.2 mm (0.40 in) on an 8.5 in rim. The tire measures about 235 mm (9.25 in).
Measuring Rim Width Comparator Conversion Table
The Measuring Rim Width Comparator conversion table adjusts section width for 6 tires mounted on rims different from the measuring rim.
| Size | Measuring rim | Actual rim | Estimated section width | Change |
|---|---|---|---|---|
| 225/45 R17 | 7.5" | 7.5" | 225.0 mm (8.86 in) | +0.0 mm |
| 225/45 R17 | 7.5" | 8.5" | 235.2 mm (9.26 in) | +10.2 mm |
| 245/40 R18 | 8.5" | 9.5" | 255.2 mm (10.05 in) | +10.2 mm |
| 205/55 R16 | 6.5" | 7.0" | 210.1 mm (8.27 in) | +5.1 mm |
| 265/70 R17 | 8.0" | 7.5" | 259.9 mm (10.23 in) | -5.1 mm |
| 255/35 R19 | 9.0" | 8.5" | 249.9 mm (9.84 in) | -5.1 mm |
What is Measuring Rim Width Comparator?
A Measuring Rim Width Comparator is a tool that explains why a tire's published dimensions differ from the dimensions on a particular wheel. Every published tire dimension is measured on one specified rim width.
Two tire makers can publish the same size on different measuring rims. Comparing published widths without the measuring rim gives a misleading result.
The 0.4 factor is the ETRTO estimate for radial passenger tires and gives about 5 mm (0.2 in) per 0.5 in of rim width.
What are the uses of Measuring Rim Width Comparator?
There are 4 main uses of the Measuring Rim Width Comparator, including data sheet comparison and clearance planning.
Data sheet comparison
Compare 2 brands' published widths on equal terms.
Clearance planning
Estimate real width on the actual wheel.
Narrow rim checks
See how much a tire narrows on a narrower winter wheel.
Stagger setups
Estimate real widths for front and rear wheels of different widths.
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