TireFitLab

TireFitLab fitment tools

Tire size calculator: see if it fits — before you buy.

Free tire size calculator: decode tire and wheel sizes, compare two setups, and see diameter, speedometer, sidewall and clearance changes before you buy.

Start with tire size Browse tools
  • Geometry-based
  • Methodology linked
  • Localized tolerances

What does a tire size calculator do?

A tire size calculator decodes a marking like 225/45R17 into diameter, width and sidewall height, compares it with a second size, and shows the exact differences: overall diameter, speedometer error, revolutions per mile and ride height. TireFitLab computes every value from tire geometry and flags whether the change stays within common tolerance rules — it is a geometry screening, not a vehicle-specific fitment approval.

Tire & wheel reference guides
Tire size calculator Compare tire sizes: diameter, speedometer error and plus/minus sizing — instant and mobile. Speedometer error calculator Calculate how a different tire size affects your speedometer and odometer reading. Wheel offset (ET) decoder Read a wheel marking: rim width, diameter, ET/offset, bolt pattern and centre bore explained. Tire & wheel reference guides Reference tables and guides for tire and wheel specifications: load index, speed rating, bolt pattern, rim compatibility.

Start by what you need to decide

Six common fitment journeys, each routed to the tool or guide that answers it fastest.

01 Alternative tire size Compare a current size with one or more candidates before buying. Next: Run the size comparison and save the result. 02 Speedometer concern Check how a diameter change affects indicated speed and distance. Next: Review the speedometer delta and methodology. 03 Wheel or spacer clearance See inner clearance, outer poke and spacer-adjusted effective ET. Next: Compare current and new wheel position. 04 Start from the door placard Use the tire size on the vehicle label as the source of truth. Next: Enter the placard size as the current size. 05 Decode a sidewall marking Turn metric or flotation markings into readable dimensions. Next: Convert the marking, then compare if needed. 06 Safety or legal confidence Use conservative inspection guidance before repair, replacement or travel. Next: Read the checklist and act on visible damage first.

Placard-first

Start from the size already approved for the car

Enter the tire size from the door placard, fuel flap or current sidewall, then compare a candidate replacement size.
Size-based only: no make/model database claim, and no physical-clearance guarantee.

Tool directory

Pick a category and jump straight to the calculator or guide you need.

Sizing & fitment

Fitment, diameter, speedometer and unit-conversion tools.

Wheels & setups

Wheel, gearing and staggered setup helpers for real-world builds.

Ownership & wear

Age, tread and running-cost checks for day-to-day decisions.

Tire & wheel reference guides

Charts and guides for markings, ratings and fitment rules.

Content updated
2026-09-04
What changed
  • Led the localized hero heading with the measured head query in every locale.
  • Retitled the homepage to the measured head query, rewrote the localized lede and added a literal answer capsule.
Methodology