Tire Size Calculator

Compare tire dimensions, speedometer errors, and fitments for custom wheels.

Tire Size Calculator Options
Tire Size 1 (Stock)
Tire Size 2 (New)
Result Details
Enter parameters and click Calculate to view.
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Automotive Tool

Tire Size Calculator

Compare tire sizes and calculate speedometer discrepancies.

100% Free & Local
STEP 01

Stock Tire Sizes

Enter width (mm), aspect ratio (%), and wheel rim diameter (inches) of your original equipment tires.

STEP 02

Proposed New Sizes

Input the sizing specifications of the alternative tires you want to compare or fit.

STEP 03

Run Diagnostics

The engine computes sidewall thickness, outer diameter ratios, and rolling circumference measurements.

STEP 04

Speedometer Check

Inspect the calculated speed offset percentage error at standard highway speeds (e.g. 60mph).

Your Privacy Matters

All calculations run locally in your browser.

No Server Uploads

No variables or results are sent to our servers.

Auto-Cleared

Your session data is erased upon closing the tab.

Capabilities

Key Capabilities

Complete Features

Detailed Side-by-Side Spec

Compares outer rolling diameter, circumference, and tire height to verify clearance limits.

Speedometer Calibration

Calculates actual vehicle velocity compared to standard speedometer readouts.

Revolutions Counter

Counts the exact tire rotation count per mile, predicting changes in gear ratios.

Fitment Clearances

Evaluates width changes to prevent tire rubbing on inner suspension struts or outer fenders.

Private Computations

No automobile configurations are logged. All comparison calculations run locally.

Help

Common Questions Answered

Support
Q1 What do the numbers on a tire sidewall mean (e.g. 205/55R16)?
205 is the section width in millimeters, 55 is the aspect ratio percentage (sidewall height is 55% of width), R indicates radial construction, and 16 is the rim diameter in inches.
Q2 Why does a different tire size affect the speedometer?
Your vehicle measures speed based on wheel revolutions. A larger tire has a greater circumference and travels further per turn, making the speedometer read slower than actual speed.
Q3 What is a safe tire diameter difference percentage?
It is generally recommended to keep the overall diameter difference within 3% of the stock tires to avoid safety, gearing, and brake system issues.
Q4 How is tire sidewall height calculated?
Sidewall height is calculated by multiplying width by aspect ratio percentage: (Width * Aspect / 100).
Q5 Can changing tire size void my vehicle warranty?
Installing tires that deviate significantly from factory specs can affect suspension components and may impact certain elements of your warranty. Check manufacturer specs first.