Effective Gear Ratio Calculator
Calculate your effective gear ratio after changing tire size so you can see the impact on RPM and gearing, free in your browser.
Drivetrain
Tyres can be a size (265/70R17) or a diameter — only the ratio of the two matters, so the unit cancels. Runs in your browser.
Result
Bigger tyres lower the effective gearing — less torque multiplication, lower cruising RPM and slower acceleration. Fitting a numerically higher (lower) diff ratio restores the stock feel.
Estimates only — check before you fit
These results are calculated from tyre-size geometry and the values you enter. Real tyre dimensions vary by brand, model, load, pressure and wear. Always check the vehicle placard, tyre manufacturer data, local road rules, and guard / brake / suspension clearance, and get professional fitment advice before changing tyre or wheel sizes. This is a calculation estimate, not a driveline recommendation.
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the stock and new tyres (a size like 265/70R17 or a diameter — units cancel).
- Enter the stock differential (gear) ratio, e.g. 4.10.
- Read the effective ratio, the ratio that restores stock, the torque change and — if you add a cruising RPM — the new RPM.
How it works
Effective ratio = stock ratio × (stock diameter ÷ new diameter). Bigger tyres reduce the effective ratio (taller gearing); the ratio to restore stock is stock ratio × (new diameter ÷ stock diameter).
Because only the ratio of the two diameters matters, you can enter sizes or diameters in any unit. This is a calculation estimate, not a driveline recommendation.
Worked example
4.10 diff, 30″ → 33″ tyres. Effective ratio ≈ 4.10 × 30/33 = 3.73; to restore the stock feel you'd want about 4.10 × 33/30 = 4.51.
Common mistakes
- Mixing units between the two tyres — keep both as sizes, or both as diameters in the same unit.
- Forgetting that taller tyres also affect speedo and braking, not just gearing.
Frequently asked questions
Why do bigger tyres feel slower?
They raise the effective gearing — the engine turns fewer times per wheel revolution, so there's less torque multiplication and lazier acceleration. A numerically higher diff ratio brings it back.
What ratio should I regear to?
The 'ratio to restore stock' is a starting point. Real gear sets come in fixed steps, so pick the nearest available ratio and confirm with a driveline specialist.
Are these results safe to fit?
They're geometry estimates only. Real tyres vary by brand, load, pressure and wear. Always check the vehicle placard, manufacturer data, local road rules and guard/brake/suspension clearance, and get professional fitment advice before changing tyre or wheel sizes.
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Cite this tool
Zerdly. (2026). Effective Gear Ratio Calculator. Retrieved from https://www.zerdly.com/tools/effective-gear-ratio-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.