Tire Size Calculator

MiniWebtool’s Tire Size Calculator compares two width, aspect-ratio, and rim-size combinations. Review diameter, circumference, sidewall height, and revolutions per mile, plus the implied speedometer change.

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Help me understand MiniWebtool's Tire Size Calculator: https://miniwebtool.com/tire-size-calculator/. What can I use it for, what inputs or settings do I need, and how do I use it and understand the output? Walk me through one simple example. Use this page as your source and answer in English.

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Compare 225/45R17 and 235/40R18: size, revs per mile, speedo

Pick an original and a new tire size to compare overall diameter, sidewall, circumference, revolutions per mile and speedometer error.

Quick Comparisons:

Plus One: 16" to 17" Plus One: 17" to 18" Plus Two: 16" to 18" Truck: 17" to 18" Same Width Upgrade

Original Tire

Width (mm)

Aspect Ratio

Rim Diameter (inches)

225/45R17

New Tire

Width (mm)

Aspect Ratio

Rim Diameter (inches)

235/40R18

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About Tire Size Calculator

Welcome to the Tire Size Calculator, a comprehensive tool for comparing tire sizes and understanding how changes affect your vehicle. Whether you are upgrading your wheels, looking for plus-size alternatives, or checking if new tires will fit, this calculator provides visual comparisons, speedometer corrections, and detailed measurements to help you make an informed decision.

At a Glance

Tire Size Calculator compares two tire sizes, such as 225/45R17 and 235/40R18, by diameter, circumference, sidewall height and revolutions per mile. It also shows the speedometer error and a fitment rating.

Tire Size Calculator: features at a glance
Feature	What you get	Good for
Two-size comparison	Pick width, aspect ratio and rim diameter for your original and new tires from dropdowns. Preset buttons fill in common plus-size swaps.	Checking whether new wheels or tires are a sensible swap
Measurements	Overall diameter, circumference, sidewall height, width, ground clearance and revolutions per mile, with differences in millimeters, inches and percent.	Comparing plus-one or plus-two sizes against the original
Speedometer correction	A table of actual speed at various speedometer readings, using actual speed = reading x (new circumference / original circumference).	Drivers who change tire size and want to know the speedometer error
Fitment rating	The diameter difference is rated Excellent (0-3%), Acceptable (3-5%) or Caution (5% and above), with a note on recalibration and clearance.	Spotting changes large enough to affect ABS or fender clearance
Tire diagrams	Both tires are drawn to proportional scale, with the outer ring as rubber and the inner circle as the rim.	Seeing the size difference at a glance

What Do Tire Size Numbers Mean?

Tire sizes follow a standardized format like 225/45R17. Here is what each number represents:

225 - Tire width in millimeters, measured from sidewall to sidewall when mounted on a specified width rim
45 - Aspect ratio, the sidewall height expressed as a percentage of the tire width. A 45 aspect ratio means the sidewall is 45% of 225mm = 101.25mm
R - Radial construction (virtually all modern passenger tires are radial)
17 - Wheel rim diameter in inches

How Overall Diameter is Calculated

The overall tire diameter is calculated by adding the rim diameter to twice the sidewall height:

Sidewall height = Width x (Aspect Ratio / 100)
Overall diameter = (2 x Sidewall height) + (Rim diameter x 25.4mm)
Circumference = Pi x Overall diameter

For example, a 225/45R17 tire: sidewall = 225 x 0.45 = 101.25mm, diameter = (2 x 101.25) + (17 x 25.4) = 634.3mm (24.97 inches).

Understanding Plus Sizing

Plus sizing is the practice of increasing wheel diameter while decreasing sidewall height to maintain approximately the same overall tire diameter. This is the most common reason people compare tire sizes.

Plus Sizing Chart

Plus Zero: Same rim size, different width or aspect ratio
Plus One: 1 inch larger rim, lower profile tire (e.g., 205/55R16 to 225/45R17)
Plus Two: 2 inches larger rim, even lower profile (e.g., 205/55R16 to 235/40R18)
Plus Three: 3 inches larger rim (e.g., 205/55R16 to 245/35R19)

Benefits of Plus Sizing

Better handling: Lower sidewalls flex less during cornering, improving response
Improved appearance: Larger wheels with lower-profile tires give a sportier look
Wider contact patch: More rubber on the road improves grip
Better braking: Wider tires can improve braking distances on dry pavement

Trade-offs of Plus Sizing

Harsher ride: Less sidewall means less cushioning over bumps
More road noise: Wider tires generate more noise on most surfaces
Higher cost: Larger wheels and low-profile tires are more expensive
Increased vulnerability: Lower sidewalls are more susceptible to pothole damage

How Tire Size Affects Your Speedometer

Your vehicle's speedometer is calibrated for the original tire size. When you change tire sizes, the speedometer reading will be affected:

Larger tires: Your speedometer will read lower than your actual speed (you are going faster than displayed)
Smaller tires: Your speedometer will read higher than your actual speed (you are going slower than displayed)

The formula is: Actual Speed = Speedometer Reading x (New Circumference / Original Circumference)

Most manufacturers recommend keeping the overall diameter within 3% of the original to minimize speedometer error and maintain proper vehicle dynamics.

How to Use This Calculator

Enter your original tire size: Select the width, aspect ratio, and rim diameter of your current tires from the dropdown menus.
Enter the new tire size: Select the width, aspect ratio, and rim diameter of the tires you want to compare.
Use quick presets: Click any preset button for common plus-size conversions to instantly populate both fields.
Click Compare: Hit the "Compare Tire Sizes" button to see the full comparison.
Review results: Examine the visual tire comparison, dimension differences, speedometer correction table, and fitment assessment.

Understanding the Results

Visual Tire Comparison

The animated tire diagrams show both tires drawn to proportional scale, making it easy to see the size difference at a glance. The outer ring represents the tire rubber while the inner circle represents the wheel rim.

Dimension Differences

The key metrics show the differences in diameter, width, sidewall height, and ground clearance between the two tire sizes, expressed in both millimeters and inches with percentage changes.

Speedometer Correction Table

The table shows what your actual speed is at various speedometer readings, so you know exactly how much to compensate when driving with different-sized tires.

Fitment Assessment

Excellent (0-3%): Within recommended range, no speedometer recalibration needed
Acceptable (3-5%): May work but consider speedometer recalibration
Caution (5%+): Significant difference, may cause clearance issues, ABS problems, or warranty concerns

Tire Size and Fuel Economy

Changing tire size can affect fuel economy in several ways:

Rolling resistance: Wider tires have more contact area, increasing rolling resistance and fuel consumption
Weight: Larger wheels are heavier, requiring more energy to accelerate and rotate
Aerodynamics: Wider tires can slightly increase aerodynamic drag
Odometer effect: Larger tires make your odometer under-count miles, making fuel economy appear better than it actually is

Common Tire Size Conversions

Compact Cars

185/65R15 to 195/60R15 (wider, similar diameter)
195/65R15 to 205/55R16 (plus one)
205/55R16 to 215/45R17 (plus one)

Sedans and SUVs

225/45R17 to 235/40R18 (plus one)
225/65R17 to 245/55R19 (plus two)
235/55R18 to 255/45R20 (plus two)

Trucks and Off-Road

265/70R17 to 275/65R18 (plus one)
275/55R20 to 285/50R22 (plus two)
265/75R16 to 285/70R17 (larger overall for off-road)

Safety Considerations

Load rating: Ensure new tires have adequate load capacity for your vehicle
Speed rating: New tires should meet or exceed the original speed rating
Clearance: Check for adequate clearance at full steering lock and suspension compression
TPMS: Tire Pressure Monitoring Systems may need recalibration after size changes
AWD vehicles: All-wheel-drive vehicles require all four tires to be the same size to prevent drivetrain damage

Frequently Asked Questions

What do tire size numbers mean?

A tire size like 225/45R17 breaks down as: 225 is the tire width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the wheel rim diameter in inches. The overall tire diameter is calculated from these numbers.

How much tire size difference is acceptable?

Most experts recommend keeping the overall diameter within 3% of the original tire size. Up to 5% may be acceptable but could require adjustments. Differences beyond 5% may cause speedometer errors, ABS issues, and potential rubbing against fender wells.

How does tire size affect my speedometer?

Larger tires cover more ground per revolution, so your speedometer will read slower than your actual speed. Smaller tires do the opposite - your speedometer will read faster than your actual speed. For example, a 3% larger tire means when your speedometer shows 60 mph, you are actually going about 61.8 mph.

Will changing tire size affect fuel economy?

Yes, larger and wider tires generally increase rolling resistance and weight, which can reduce fuel economy by 1-3%. Conversely, the apparent fuel economy on your odometer may seem to improve because the odometer under-counts miles with larger tires. Narrower tires with similar diameter typically have better fuel efficiency.

What is the plus sizing concept for tires?

Plus sizing means increasing the wheel diameter while decreasing the sidewall height to maintain the same overall tire diameter. For example, going from 205/55R16 to 225/45R17 (plus one) keeps roughly the same diameter while giving a wider tread and lower profile for improved handling and appearance.

Additional Resources

Tire Code - Wikipedia

Reference this content, page, or tool as:

"Tire Size Calculator" at https://MiniWebtool.com/tire-size-calculator/ from MiniWebtool, https://MiniWebtool.com/

by miniwebtool team. Updated: Mar 24, 2026

Developer API available: Run this tool from your app, automation, or agent with one JSON HTTP request. View API docs

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