Tire Size Gear Ratio
Calculator
Find the reduction needed to preserve engine RPM per road speed after a tire-diameter change, plus the gearing effect if the axle ratio stays the same.
Your setup
Your results
CalculatedLarger tires need a numerically higher reduction to restore the original effective gearing.
Uses geometric diameter, not measured loaded rolling circumference. Does not validate available axle gears.
How the calculation works
Follow the formula and keep the input units consistent at every step.
new ratio = original ratio × new diameter / original diameter- Find the new-to-old tire diameter multiplier.
- Multiply the original reduction by that multiplier.
- Compare fixed-RPM speed if the original reduction is retained.
A Worked Example
Going from 26 to 30 inches with a 4.10:1 axle requires 4.10 × 30/26 = 4.73:1 to preserve engine RPM at equal road speed.
What Your Result Means
Larger tires need a numerically higher reduction to restore the original effective gearing.
Assumptions & Limitations
Uses geometric diameter, not measured loaded rolling circumference. Does not validate available axle gears.
Common Mistakes
Do not multiply by old/new diameter; that reverses the correction.
Questions about this calculation
Which values should I enter?
Find the new-to-old tire diameter multiplier. Do not multiply by old/new diameter; that reverses the correction.
How should I interpret the result’s precision?
Displayed results are rounded for readability. Intermediate calculations retain floating-point precision; extra displayed decimals do not improve your input accuracy.
Can I use metric units?
Yes. Select Metric above the calculator to convert your inputs. Results always state their units. Tire-size designations keep their conventional mm and inch units.
What are the main limitations?
Uses geometric diameter, not measured loaded rolling circumference. Does not validate available axle gears.
Method, sources & relevant guides
Angular speed and tangential speed. Gear reductions follow tooth engagement and conservation of rotation count.
OpenStax — University Physics: rotation with constant angular accelerationMethod documentation updated October 18, 2026. Independent professional review has not been claimed. Read the calculation and review policy.