Equivalent Tire Diameter Finder
Find sizes that are about as tall as yours. Use the calculator below. Results update as you type.
Overall Diameter = (Rim Diameter × 25.4) + 2 × (Width × Aspect Ratio / 100)How the Equivalent Tire Diameter Finder Works
This finder works by testing every common width and aspect ratio on several rim sizes and listing the sizes closest to a target diameter. It ranks results by the smallest gap.
- Enter the target diameter, for example 26.0 in (660 mm), or a size code to copy its diameter.
- Select the rim sizes to search, for example 16 in to 20 in.
- Read the ranked list: 185/55R18 at 26.01 in, 195/65R16 at 25.98 in, and more.
- Narrow the list by width to keep only sizes that suit your wheel.
The Formula
The formula ranks each candidate by Gap = |Candidate Diameter − Target Diameter|.
Overall Diameter = (Rim Diameter × 25.4) + 2 × (Width × Aspect Ratio ÷ 100) Gap (in) = |D_candidate − D_target| Gap (%) = (D_candidate − D_target) ÷ D_target × 100Worked example: 225/45R18 has D = (18 × 25.4) + 2 × 101.25 = 659.7 mm (25.97 in). Against a 26.0 in target, the gap is −0.03 in, or −0.11%.
Conversion Table
The conversion table lists the 12 sizes closest to a 26.0 in (660 mm) target across 16 in to 20 in rims. All 12 sit within ±0.27%.
The list mixes narrow and wide tires. A 185/55R18 and a 325/35R17 both match the target height, but they suit very different wheels. Filter by width after you find the height match.
| Size | Diameter (in) | Gap (in) | Gap (%) |
|---|---|---|---|
| 185/55R18 | 26.01 | +0.01 in | +0.05% |
| 195/65R16 | 25.98 | -0.02 in | -0.08% |
| 285/40R17 | 25.98 | -0.02 in | -0.09% |
| 255/30R20 | 26.02 | +0.02 in | +0.09% |
| 225/45R18 | 25.97 | -0.03 in | -0.11% |
| 255/35R19 | 26.03 | +0.03 in | +0.11% |
| 255/40R18 | 26.03 | +0.03 in | +0.12% |
| 295/30R19 | 25.97 | -0.03 in | -0.12% |
| 255/45R17 | 26.04 | +0.04 in | +0.14% |
| 255/50R16 | 26.04 | +0.04 in | +0.15% |
| 325/35R17 | 25.96 | -0.04 in | -0.17% |
| 205/50R18 | 26.07 | +0.07 in | +0.27% |
What Is the Equivalent Tire Diameter Finder?
The Equivalent Tire Diameter Finder is a search tool that finds tire sizes with almost the same overall diameter as a target. It covers every common width from 185 mm to 335 mm and aspect ratios from 30 to 85.
Keeping diameter constant keeps speedometer accuracy, gearing and ABS calibration close to factory. The finder shows every way to reach that diameter, including sizes a tire shop rarely suggests.
Common Uses
This finder has 4 uses: finding substitutes, planning wheel changes, locating rare sizes and matching spares.
- Find substitutes: List sizes with the same height when the factory size is out of stock.
- Plan wheel changes: See sizes that keep height on a new rim diameter.
- Locate rare sizes: Discover uncommon sizes that match exactly.
- Match spares: Find a full-size spare close to the road tires.