Auto Tools

Effective Gear Ratio Calculator

Estimate effective gearing after a tire-size change from the original rear gear ratio and old versus new tire diameter.

Updated April 15, 2026
Free online tool
Planning and research use

Why this page exists

Tire changes are easier to understand when old and new tire diameters turn an original rear gear into an effective new ratio instead of leaving the gearing change to guesswork. This calculator helps visitors estimate effective gear ratio after a tire-size change and adds a shorter-versus-taller gearing summary.

Interactive tool

Run the estimate

Enter your numbers and read the result first, then use the sections below to understand what affects the outcome.

Calculator

Effective gear ratio calculator

Estimate effective gearing after a tire-diameter change using the original rear gear ratio and old versus new tire diameter.

Result

3.83

Estimated effective rear gear ratio after a tire-diameter change using the original gear ratio and old versus new tire size.

Effective new gear ratio3.83

Original ratio used4.10

Original tire diameter28.00

New tire diameter30.00

Gearing summaryTaller effective gearing than before

A move from 28.00 to 30.00 in tire diameter changes an original 4.10 ratio to an effective ratio near 3.83.
Taller effective gearing than before, which is about -6.67% versus the original ratio.
Use the result as a tire-change planning estimate only, because real RPM change also depends on transmission ratios, tire growth, and slip.

This is a planning estimate only. Real gearing feel can also change with converter slip, transmission ratios, tire growth, and vehicle setup.

Planning note

Last updated April 15, 2026. Use this tool to compare scenarios and plan ahead, then confirm important details with the lender, employer, insurer, contractor, or other qualified provider involved in the final decision.

How it works

What the calculator is doing

Enter the original rear gear ratio.

Enter the original tire diameter and the new tire diameter using the same unit for both.

The calculator adjusts the original ratio by the tire-diameter change and shows the effective ratio.

Understanding your result

This is a tire-and-gear planning estimate, not a full driveline model. Converter slip, transmission ratios, tire growth, and vehicle setup can all affect the real on-road result.

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Examples

Ways people use this tool

Example scenarios help turn a quick estimate into a more useful comparison or planning step.

See how taller tires change gearing

A larger tire can make the effective ratio lower, which may feel like taller gearing on the road.

Compare two tire choices before swapping

A quick ratio estimate can make it easier to compare how two tire sizes may change the effective gearing feel.

Use it with driveline tools

Effective gear ratio often fits naturally beside speedometer, trap-RPM, and tire-diameter tools.

FAQ

Common questions

How is effective gear ratio estimated here?

The calculator multiplies the original rear gear ratio by the original tire diameter divided by the new tire diameter.

Why do both tire diameters need the same unit?

Because the formula compares the two diameters directly, so they need to be expressed on the same scale.

What does taller versus shorter gearing mean here?

A lower effective numerical ratio is usually described as taller gearing, while a higher effective numerical ratio is usually described as shorter gearing.

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Keep comparing

Use these related tools to compare nearby scenarios, check a second estimate, or keep narrowing down the right decision.

Auto ToolsUpdated April 12, 2026