Rim Diameter Comparator
Compare the rim diameter two tire sizes need. Convert wheel sizes from inches to mm and see how much sidewall a larger wheel removes.
Compare required wheel diameters.
Rim Diameter Comparator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Rim Diameter Comparator Works?
The Rim Diameter Comparator works by reading the rim number from each tire size, converting both values to millimeters and showing the sidewall change needed to hold the same overall diameter. The tool uses 3 steps.
Enter both tire sizes
Type the current size, for example 225/45 R17, and the new size, for example 225/40 R18.
Convert the rims
The comparator converts 17 in to 431.8 mm and 18 in to 457.2 mm.
Show the bead seat change
The comparator reports a 25.4 mm (1.0 in) larger bead seat diameter and 12.7 mm (0.5 in) less sidewall needed per side to keep the same height.
Rim Diameter Comparator Formula
The Rim Diameter Comparator formula is ΔRim_mm = (Rim₂ − Rim₁) × 25.4, and the sidewall offset per side is ΔRim_mm ÷ 2.
D_rim = Rim × 25.4ΔRim_mm = (Rim₂ − Rim₁) × 25.4ΔSH_target = −ΔRim_mm ÷ 2- Rim₁, Rim₂
- Nominal rim diameter of current and new wheel (in)
- D_rim
- Bead seat diameter (mm)
- ΔSH_target
- Sidewall change that keeps overall diameter fixed (mm)
Worked example
A move from a 17-inch to a 19-inch wheel raises the bead seat diameter by 50.8 mm (2.0 in). Each sidewall needs to shrink by 25.4 mm (1.0 in) to keep the overall diameter unchanged.
Rim Diameter Comparator Conversion Table
The Rim Diameter Comparator conversion table converts 11 standard wheel sizes from inches into millimeters and centimeters and lists the sidewall change that offsets each size against a 17-inch wheel.
| Rim size | Millimeters | Centimeters | Change vs 17 in | Sidewall change for equal OD |
|---|---|---|---|---|
| 14 in | 355.6 mm | 35.6 cm | -76.2 mm | +38.1 mm |
| 15 in | 381.0 mm | 38.1 cm | -50.8 mm | +25.4 mm |
| 16 in | 406.4 mm | 40.6 cm | -25.4 mm | +12.7 mm |
| 17 in | 431.8 mm | 43.2 cm | +0.0 mm | +0.0 mm |
| 18 in | 457.2 mm | 45.7 cm | +25.4 mm | -12.7 mm |
| 19 in | 482.6 mm | 48.3 cm | +50.8 mm | -25.4 mm |
| 20 in | 508.0 mm | 50.8 cm | +76.2 mm | -38.1 mm |
| 21 in | 533.4 mm | 53.3 cm | +101.6 mm | -50.8 mm |
| 22 in | 558.8 mm | 55.9 cm | +127.0 mm | -63.5 mm |
| 23 in | 584.2 mm | 58.4 cm | +152.4 mm | -76.2 mm |
| 24 in | 609.6 mm | 61.0 cm | +177.8 mm | -88.9 mm |
What is Rim Diameter Comparator?
A Rim Diameter Comparator is a tool that compares the wheel diameters two tire sizes fit and converts the difference into millimeters of bead seat and sidewall. The rim number is the last number in a metric size, for example 18 in 245/40 R18.
A tire only mounts on a wheel with the exact same nominal rim diameter. A 17-inch tire never fits an 18-inch wheel, and the comparator makes that mismatch obvious before purchase.
Half-inch sizes such as 16.5 in and 19.5 in exist for older light trucks and commercial vehicles. Those rims use a different bead seat profile and do not accept 16-inch or 19-inch tires.
What are the uses of Rim Diameter Comparator?
There are 4 main uses of the Rim Diameter Comparator, including wheel upgrade planning and brake clearance checks.
Wheel upgrade planning
Confirm the new wheel matches the rim number of the chosen tire.
Sidewall budgeting
Find how much sidewall a larger wheel removes.
Brake clearance checks
Check that a smaller winter wheel still clears large brake calipers.
Ordering mistakes
Catch a wheel and tire order with mismatched rim numbers.
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