Practical tools for measured work
Tire Size Comparison Calculator
Compare two tyre sizes side by side - diameter, sidewall, circumference and speedometer effect.
e.g. 225/45R18
e.g. 245/40R18
How it works
Each tyre's overall diameter = wheel diameter + 2 × (section width × aspect ratio). The speed error assumes tyre B is fitted but the speedometer remains calibrated for tyre A - actual speed = indicated speed × (diameter B ÷ diameter A).
Worked example: compare the entered original and proposed sizes, then use the diameter-change percentage to judge gearing and clearance while the speed result shows what the vehicle is actually doing at the displayed reference speed.
Tyre dimensions are nominal. Measure the fitted tyre when clearance is close because rim width, pressure, load and manufacturer construction change the real section width and diameter.
FAQ
Why do the wheel diameters need to match?
Related tools
Wheel Offset Calculator
Compare a current and proposed wheel/tyre setup and see exactly how the geometry changes.
Wheel Spacer Calculator
See how a wheel spacer changes your effective offset and how far the wheel moves outward.
Scrub Radius Calculator
See how much a wheel offset or spacer change shifts your scrub radius from a known baseline.
Gear & RPM Planner
Compare road speed and engine RPM across six transmission gears.