Target Overall Diameter Calculator

This calculator works by solving the aspect ratio that gives a chosen diameter on a chosen width and rim. It returns the exact ratio and the closest real size.

1. Enter the target diameter, for example 30.5 in (775 mm).
2. Enter the width, for example 265 mm.
3. Enter the rim, for example 17 in.
4. Read the exact aspect ratio (64.7%) and the closest size: 265/65R17 at 30.56 in.

The formula is Aspect Ratio = ((Target Diameter − Rim) × 25.4 ÷ 2) ÷ Width × 100.

Worked example: target 32.0 in, width 275 mm, rim 20 in. Sidewall = (32 − 20) × 25.4 ÷ 2 = 152.4 mm. Aspect = 152.4 ÷ 275 × 100 = 55.4%, so the closest size is 275/55R20 at 31.91 in.

The conversion table solves 10 target diameters from 25.0 in to 33.0 in. Each row shows the exact aspect ratio, the closest size and that size's real diameter.

Rounding causes the gap between the target and real diameter. A 31.5 in target on a 275 mm, 18 in combination solves to 62.3%; the 60 series gives 30.99 in.

The Target Overall Diameter Calculator is a tool that finds the aspect ratio needed to hit a specific tire height on a given width and wheel. It is the reverse of the tire diameter calculator.

The tool suits owners who know the height they need, such as 33 in for a lift kit or 28.5 in to match the factory tire, and the wheel they own.

This calculator has 4 uses: lift sizing, factory matching, wheel planning and custom builds.

• Size lift tires: Solve the profile for a 33 in or 35 in target.
• Match factory height: Hold the original diameter on a new width.
• Plan wheel purchases: Check if a size exists for a wheel you want.
• Design custom builds: Hit exact heights for show or race vehicles.