TireFitLab fitment tools

Tire size calculator

Compare two sizes. See the changes in diameter, sidewall, ride height and speedometer readings — calculated from tire geometry.

Compare sizes Browse tools
  • Geometry-based
  • Methodology linked
Compare sizes Worked example
Current 225/45R17
New 235/40R18

Diameter: 634.3 mm. Sidewall 101.25 mm

Diameter: 645.2 mm. Sidewall 94 mm

Δ Diameter
+1.7%
Δ Width
+10 mm
Δ Ride-height change
+5.45 mm

Nominal geometry. Vehicle fitment must be verified.

Browse tools

Tire size calculator

Compare diameter, sidewall and ride-height changes.

Speedometer error calculator

See how tire size changes speed and distance readings.

Wheel offset (ET) decoder

Decode rim width, diameter and ET markings.

Tire & wheel reference guides

Look up load indices, speed ratings and bolt patterns.

What would you like to check?

Choose a task to find the right calculator or guide.

Quick comparison

Start with the vehicle placard

Enter the listed tire size and the replacement size you want to compare.

Size comparison only. Vehicle fitment and clearance need separate checks.

Tool directory

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

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.