How does changing tire size affect my speedometer?

Larger tires have a greater circumference, meaning they travel further per revolution, which causes your speedometer to read slower than your actual speed.

What is the effective final drive ratio?

It is the adjusted gear ratio resulting from a tire size change, showing how the new tire diameter alters your vehicle's mechanical leverage compared to the original setup.

Can I input reverse gear ratios in the calculator?

Yes, you can include your reverse gear ratio by prefixing the value with the letter 'R' (e.g., R3.8).

Does this calculator support both metric and imperial units?

Yes, you can toggle between metric units (km/h, mm) and imperial units (mph, inches) for all inputs and outputs.

Are my vehicle specifications sent to a server?

No, all calculations are performed locally in your web browser.

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Tire Size & Gear Ratio Calculator

Enter a tire size (285/70R17), differential/final-drive ratio, and transmission gear ratios to compute the true overall tire diameter, circumference, revolutions per mile/km, an RPM × gear speed table, effective final drive after a tire change, and speedometer error. Metric or imperial units, with optional before/after tire comparison.

Details

What this tool helps you do

A drivetrain calculator for car, truck, and 4×4 modifiers:

Tire size — enter a P-metric spec like 285/70R17. Computes sidewall, diameter, circumference, revolutions per mile/km.
Final drive — your differential / axle ratio (e.g. 4.10).
Transmission gears — one ratio per line/comma; prefix R for reverse.
RPM table — a matrix of speed in every gear at every RPM.
Tire comparison — diameter change, effective final drive, speedometer error, actual speed.

All math runs locally.

Execution

Run this tool

Fill in the form, run the tool, and review the result in one place.

Prepared example runs

Click an example to fill the form automatically. File inputs still need an upload.

2 examples

Lift truck: 285/70R17 with 4.10 diffs

A common 4×4 tire+gear combo — see diameter, the RPM×gear speed table, and cruise RPM at 100 km/h.

<div class="tg-wrap"><section><h3>🛞 Tire dimensions</h3><table>…diameter ≈ 32.8 in…</table></section><table class="tg-matrix">…speed per gear×RPM…</table></div>

Sports car: 245/45R18, imperial units

A low-profile performance setup in mph — check RPM at a 70 mph highway cruise in top gear.

<div class="tg-wrap"><section><h3>🛞 Tire dimensions</h3>…diameter ≈ 25.7 in…</section><p class="tg-hint">At 70 mph in top gear… engine ≈ …RPM.</p></div>

Inputs

Set the required fields, then run the tool.

7 options

ContentPaste or type the main input values.4

Tire size (e.g. 285/70R17)textRequiredFull P-metric tire size, e.g. 285/70R17, 265/75R16, 245/45R18.Transmission gears (one ratio per line, e.g. 3.6,2.1,1.4,1,0.8,R3.8)textareaRequiredGear ratios in order; prefix with R for reverse (e.g. R3.8). Comma or newline separated.Engine RPM for tabletextOptionalComma-separated RPM values for the speed matrix columns.Original tire (for comparison, optional)textOptionalOptional original tire size for before/after comparison.

SettingsAdjust formats, ranges, numbers, and modes.3

Differential (final drive) rationumberRequiredUnitsselectOptionalReference speednumberRequiredSpeed used for the cruise-RPM and speedometer-error calculations (in your selected unit).

Result

Ready for a run

Run the tool to preview files, text, structured data, or streamed output here.

Samples

Tire Size & Gear Ratio Calculator

What this tool helps you do

Run this tool

Prepared example runs

Inputs

Result

Examples that match this tool

Continue with connected tools and hubs

Prepared example runs

Inputs

Result

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Optimizing a Lifted 4x4 Truck Drivetrain

2. Checking Highway Cruise RPM for a Sports Car

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