Tire Width Difference Calculator

This calculator works by subtracting the old section width from the new one and splitting the change evenly between the inner and outer sides. It reports millimeters, inches and percent.

1. Enter the current width, for example 245 mm.
2. Enter the new width, for example 275 mm.
3. Read the change: +30 mm (+1.18 in), or +12.2%.
4. Read the per-side change: +15 mm toward the strut and +15 mm toward the fender.

The per-side split assumes the same wheel and offset. A different wheel shifts the split.

The formula is Width Change = New Width − Old Width. Each side moves by half of that change.

Worked example for 215 mm to 235 mm: change is +20 mm (+0.79 in), +9.3%, and 10 mm per side.

The conversion table lists 10 width upgrades with the change in mm, inches, percent and per side. Upgrades range from 10 mm (+0.39 in) to 30 mm (+1.18 in).

The per-side column is the number to compare with measured clearance. A 30 mm upgrade needs 15 mm (0.59 in) of free space on each side.

The Tire Width Difference Calculator is a tool that measures the change in section width between 2 tires and how that change splits between the inner and outer sides. Width changes affect grip, steering feel, rim fit and clearance.

Wider tires put more rubber on the road and raise rolling resistance. Each 10 mm width step moves the approved rim range up by about 0.5 in (12.7 mm).

This calculator has 4 uses: inner clearance checks, fender checks, rim checks and grip planning.

• Check inner clearance: Compare the per-side change with space at the strut.
• Check fender clearance: Compare it with space at the fender lip.
• Check rim fit: Confirm the new width suits the rim width.
• Plan grip changes: Size the contact patch for track or winter use.