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Tire Size Comparison & Speedometer Calculator

Compare tire diameters, sidewall heights, and speedometer differences.

Quick Answer:Tire code format (e.g. 225/45R17) specifies section width (mm), aspect ratio (% of width as sidewall), and wheel rim diameter (inches).

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How to Use the Tire Size Calculator: Compare Diameter & Speedometer

1Enter the width, aspect ratio, and wheel diameter of your original tire (e.g. 225 / 45 / R17).
2Input the dimensions of your proposed replacement tire (e.g. 245 / 40 / R18).
3Review the diameter difference percentage and revolutions per mile.
4Check that diameter variance remains within the recommended 3.0% safety threshold.

Formula & How It Works

Tire Diameter & Speedometer Equations

Diameter = Rim + 2 × (Width × Aspect% / 25.4) | Speedo Error = 60 × (New Diam / Old Diam)

Calculates overall diameter from sidewall and rim dimensions.

Worked Examples

Upgrading 225/45R17 to 245/40R18

Input: Tire 1: 225/45R17, Tire 2: 245/40R18

Explanation: The larger tire travels slightly farther per revolution, causing your vehicle to travel at 61.8 mph when the speedometer reads 60 mph.

Result: Diameter 1: 24.97", Diameter 2: 25.72" (+3.0%), Speedo at 60: 61.8 mph

Frequently Asked Questions

What is the maximum safe tire diameter difference when upsizing?▼

Automotive engineers generally recommend keeping overall tire diameter changes within ±3.0% of the factory original size to maintain accurate braking and speedometer safety.

What does the 'R' mean in a tire size like 225/45R17?▼

The 'R' designates Radial construction, meaning internal fabric cords run radially at 90 degrees to the direction of vehicle travel.

Will a larger tire make my car accelerate faster or slower?▼

A larger overall tire diameter effectively acts as a taller final drive gear ratio, slightly reducing acceleration torque off the line while potentially lowering highway cruising RPM.

Does tire wear change speedometer accuracy?▼

Yes. A tire worn from 10/32" down to 2/32" tread depth loses roughly 0.5 inches in diameter, causing the speedometer to read roughly 1.5% faster than actual speed.

Quick Reference & Conversion Table

Tire Size	Overall Diameter	Sidewall Height	Revs / Mile
205/55R16	24.88 in	4.44 in	811 revs
225/45R17	24.97 in	3.99 in	808 revs
245/40R18	25.72 in	3.86 in	785 revs
275/35R19	26.58 in	3.79 in	759 revs

⚠️ Common Pitfalls to Avoid

•Exceeding a 3% diameter change, which can trigger ABS brake faults, traction control errors, and transmission shift irregularities.
•Ignoring wheel width (rim width) compatibility and tire clearance inside the fender well.

💡 Practical Real-World Uses

•Choosing aftermarket wheels and plus-size tire packages for passenger cars and trucks.
•Recalibrating electronic speedometer modules after installing oversized off-road tires.

Last reviewed & updated: 2026-11-11

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