How do I read a tire size like 225/65R17?

The first number (225) is the section width in millimeters. The second number (65) is the aspect ratio — the sidewall height as a percentage of the width. 'R' means radial construction. The last number (17) is the rim diameter in inches. So a 225/65R17 tire is 225 mm wide, with a sidewall height of 146.25 mm (65% of 225), designed for a 17-inch wheel.

How does tire size affect speedometer accuracy?

Your speedometer is calibrated for the original tire circumference. A larger tire covers more ground per revolution, so your actual speed is higher than the speedometer reads. A 3% increase in diameter means at 60 mph indicated, you're actually going about 61.8 mph. Our speedometer table shows the exact error at every common speed.

Can I put a different size tire on my car?

Yes, but stay within the ±3% diameter tolerance and ensure the tire physically fits without rubbing on fenders, suspension, or brake components. You should also match the load index and speed rating of your original tires. For AWD vehicles, all four tires should be the same circumference to avoid drivetrain damage.

What is a plus-one (or plus-two) tire upgrade?

A plus-one upgrade increases the rim diameter by 1 inch while reducing the aspect ratio to maintain approximately the same overall tire diameter. For example, going from 225/65R17 to 225/55R18. This gives a wider contact patch and better handling without affecting the speedometer or clearance.

How does tire size affect fuel economy?

Larger and wider tires generally decrease fuel economy due to increased rolling resistance, more weight, and greater aerodynamic drag. A significant tire size increase (e.g., going from 225 mm to 285 mm width) can reduce fuel economy by 1–3 mpg. Taller tires also lower engine RPM at highway speeds, which can partially offset the efficiency loss.

How do revolutions per mile relate to tire size?

Revolutions per mile is the number of times a tire rotates to cover one mile. Larger tires have fewer revolutions per mile because each rotation covers more ground. This directly affects odometer and speedometer readings — fewer revs per mile means the odometer under-reports actual distance traveled.

Can I use a narrower tire for better fuel economy?

Yes, going to a narrower tire (e.g., from 245 to 225 mm) reduces rolling resistance and aerodynamic drag, which can improve fuel economy. However, ensure the narrower tire is approved for your wheel width and that it maintains adequate load capacity for your vehicle. Check your vehicle owner's manual for approved tire size ranges.

Tire Size Calculator

Units

Original Tire

Tire Code (e.g. 225/65R17 or 31x10.50R15)

225/65R17

Width (mm)

Aspect (%)

Rim (in)

New Tire

Tire Code (e.g. 225/65R17 or 31x10.50R15)

235/65R17

Width (mm)

Aspect (%)

Rim (in)

Diameter Difference

+0.51in

+1.8% change

225/65R17

→

235/65R17

At 60 mph indicated, your actual speed is 61.1 mph — faster by 1.1 mph.

Size Comparison

Visual side-by-side of original vs new tire

Within Safe Range (±3%)

Compatible with ABS, traction control, and speedometer

+1.8%

diameter change

What This Means

Key implications of this tire size change

Diameter increased

Your new tire is 0.51" larger in overall diameter (1.8% bigger).

Ground Clearance

Raises your vehicle by 0.26" per side — minor change, usually safe.

Speedometer Impact

At 60 mph on your speedometer, you're actually traveling at 61.1 mph — faster by 1.1 mph. This means you may be speeding without realizing it.

3% Rule

The diameter change is 1.8% — within the recommended ±3% OEM tolerance. Safe for ABS, traction control, and stability systems.

Tire Dimensions

Side-by-side comparison of all measurements

Original

Diameter

28.52in

Width

8.86in

Sidewall

5.76in

Circumference

89.58in

Revs/mile

707

New

Diameter

29.03in

Width

9.25in

Sidewall

6.01in

Circumference

91.19in

Revs/mile

695

Clearance

+0.26in

Revs/mi

-12

Width

+10mm

Actual speed vs indicated speed with new tires

Indicated

Actual

How to Use This Tire Size Calculator

Two tire sizes, five steps — here's how to compare them

1
Enter your original tire size. Type a tire code like 225/65R17 or 31×10.50R15 in the text box, or use the width/aspect/rim dropdowns below it. The calculator accepts both metric and flotation (inch) formats.
2
Enter the new tire size you're considering. The same text-box or dropdown entry works for the new size. If you enter a flotation size like 33×12.50R17, the calculator also shows the approximate metric equivalent (e.g., ≈ 315/65R17).
3
Review the diameter difference. The hero card at the top shows the change in overall diameter (inches and millimeters), the percentage difference, and a safety badge. A green shield means you're within the safe ±3% OEM tolerance. An amber warning means the change may affect ABS, traction control, and speedometer accuracy.
4
Check the “What This Means” card. It gives you plain-language explanations of the diameter change, ground clearance impact, speedometer error at 60 mph (or 90 km/h in metric mode), and whether the change respects the ±3% rule — all in one place.
5
Use the speedometer chart. The bar chart at the bottom of the results shows your actual speed versus indicated speed at common driving speeds — from 20 to 90 mph (or 30 to 170 km/h). Hover over any bar for exact numbers, or switch between imperial and metric units with the toggle.

What is a Tire Size Calculator?

Understand what the tool does and why it matters

A tire size calculator converts the metric tire code (e.g., 225/65R17) into real-world dimensions — overall diameter, sidewall height, circumference, and revolutions per mile — and compares two tire sizes side by side. This helps you understand how changing tire size affects ride height, speedometer accuracy, ground clearance, and vehicle handling before you buy.

Tire Dimensions

Diameter, width, sidewall, and circumference from any tire code

Speedometer Error

See how tire changes affect your speedometer reading

Safety Check

±3% OEM diameter tolerance for ABS and traction control

How Tire Size is Calculated

Understanding the formulas behind tire dimensions

Sidewall Height

Sidewall = Width (mm) × Aspect Ratio / 100

Overall Diameter

Diameter = Rim (in) + 2 × Sidewall (in)

Circumference

Circumference = π × Overall Diameter

Revolutions / Mile

Revs/mi = 63,360 / Circumference (in)

Reading a Tire Size Code

P 225 / 65 R 17

Code	Meaning
P	Passenger vehicle (P-metric). LT = Light Truck, ST = Special Trailer.
225	Section width in millimeters — the tire's width from sidewall to sidewall.
65	Aspect ratio — sidewall height as a percentage of the section width.
R	Radial construction — the standard for modern tires.
17	Rim diameter in inches — the wheel size the tire fits.

Safe vs Unsafe Tire Size Changes

When changing tire size is OK and when it becomes risky

Most vehicle manufacturers recommend staying within ±3% of the original tire's overall diameter. Going beyond this range can cause issues with ABS, traction control, stability systems, and speedometer accuracy.

Safe Changes

Within ±3% diameter of OEM spec

Plus-one rim upgrade with matching aspect ratio

Same load index and speed rating

All four tires same circumference (AWD)

Risky Changes

More than 3% diameter difference

Different front/rear sizes on AWD

Lower load index than OEM spec

Rubbing on fenders at full lock or bumps

Popular Tire Swap Examples

Plus-One Rim Upgrade

within 3%

225/65R17 → 225/55R18

Same diameter, better handling

Wider Tire, Same Rim

within 3%

225/65R17 → 235/65R17

+0.6" diameter, wider contact

Off-Road Upgrade

check fit

265/70R17 → 285/70R17

+1.1" diameter, may need lift

Fuel Economy Swap

within 3%

245/45R18 → 225/50R18

Narrower, less rolling resistance

Tips for Changing Tire Size

Practical advice to avoid costly mistakes

Recalibrate Your Speedometer

A 4% diameter increase means your speedo reads 60 mph when you're actually going 62.4 mph.

Safety

Stay Within ±3%

Keep the overall diameter within 3% of OEM to maintain ABS and traction control accuracy.

Critical

Check Fender Clearance

Larger tires may rub on fender wells or suspension, especially at full steering lock.

Fitment

Match Load Ratings

Wider or different-profile tires may have lower load index, which is dangerous for heavy vehicles.

Load

AWD: Match All Four

Different circumferences front vs rear cause drivetrain stress on all-wheel-drive vehicles.

Drivetrain

Consider Fuel Economy

Larger, heavier tires increase rolling resistance, reducing fuel efficiency by 1–3 mpg.

Efficiency

Worked Examples

Real examples of common tire size comparisons

225/65R17 → 235/65R17

Original: 225/65R17 — Diameter = 28.5", Sidewall = 5.75", Circumference = 89.6"

New: 235/65R17 — Diameter = 29.0", Sidewall = 6.0", Circumference = 91.2"

Result: +0.5" diameter (+1.8%), +0.25" ground clearance. Within the safe ±3% range.

Speedometer at 60 mph: Actual speed ≈ 60.9 mph.

265/70R17 → 285/70R17

Original: 265/70R17 — Diameter = 31.6", Sidewall = 7.3", Circumference = 99.3"

New: 285/70R17 — Diameter = 32.7", Sidewall = 7.9", Circumference = 102.7"

Result: +1.1" diameter (+3.5%), +0.55" ground clearance. Exceeds the 3% tolerance — may require a lift kit.

Speedometer at 60 mph: Actual speed ≈ 62.1 mph.

33x12.50R17 (Flotation Size)

Inch size: 33x12.50R17 — Diameter = 33.0", Width = 12.50", Rim = 17"

Approximate metric: ≈ 315/65R17 (width ≈ 315 mm, aspect ≈ 65%)

Note: Flotation sizes like 33x12.50R17 are popular on lifted trucks and off-road vehicles. The metric conversion is an approximation — always verify fitment with your tire shop.

How to Calculate Tire Diameter from a Tire Code

Let's calculate the diameter for 225/65R17:

Sidewall height: 225 mm × 65 / 100 = 146.25 mm
Sidewall in inches: 146.25 ÷ 25.4 = 5.76"
Overall diameter: 17" rim + 2 × 5.76" = 28.5"

This formula works for any metric tire code: Diameter = Rim + 2 × (Width × Aspect / 100 / 25.4)

Can This Calculator Tell What Size Tires Fit My Car?

Understanding what this tool does — and what it doesn't

This calculator compares two tire sizes mathematically — it shows diameter difference, speedometer error, and whether the change stays within the ±3% OEM tolerance. It does not contain a vehicle fitment database and cannot tell you which specific tire sizes are approved for your make and model.

What This Calculator Does

Check if a new tire size stays within ±3% diameter of OEM for ABS and traction control safety

Show speedometer error at every common speed from 20–90 mph (or 30–170 km/h)

Compare diameter, sidewall height, circumference, revs per mile, and ground clearance

Show the approximate metric equivalent when you enter a flotation size like 33×12.50R17

What This Calculator Doesn't Do

Tell you which tire sizes are approved for your specific make, model, and year

Replace the vehicle owner's manual or tire placard on your driver's door jamb

Guarantee a size within ±3% will physically clear your fenders, suspension, and brakes

Substitute for a professional tire shop fitment check and mounting

A vehicle-specific fitment database is on our roadmap. For now, use this calculator to verify diameter compatibility, then confirm physical fitment with your tire dealer using real-world measurements.

Frequently Asked Questions

Common questions about tire sizes, comparisons, and compatibility

To calculate tire size, you need three numbers from the tire code: Section Width (mm), Aspect Ratio (%), and Rim Diameter (inches).

Step 1: Calculate sidewall height = Width × Aspect / 100. Step 2: Convert to inches by dividing by 25.4. Step 3: Add twice the sidewall height to the rim diameter. The formula is: Diameter = Rim + 2 × (Width × Aspect / 100 / 25.4).

For a 225/65R17: sidewall = 146.25 mm = 5.76 inches, diameter = 17 + 2 × 5.76 = 28.5 inches.

Tire diameter depends on the width, aspect ratio, and rim size encoded in the tire code. Multiply the section width (mm) by the aspect ratio (as a decimal) to get the sidewall height in millimeters. Divide by 25.4 to convert to inches. Then: Diameter = Rim Diameter + 2 × Sidewall Height (in).

Example for 225/65R17: sidewall = 225 × 0.65 / 25.4 = 5.76". Diameter = 17 + 2 × 5.76 = 28.5". Use our calculator above to do this instantly — just type any tire code.

There are two tire sizing systems: Metric (e.g., 225/65R17) and Flotation/Inch (e.g., 31x10.50R15). For metric sizes, convert using the formula: Diameter (in) = Rim + 2 × (Width × Aspect / 2540). For a 225/65R17: 17 + 2 × (225 × 65 / 2540) = 28.5".

For flotation sizes, the diameter is given directly — 31x10.50R15 means 31" diameter, 10.50" width, 15" rim. Our calculator accepts both formats — type a metric code like 225/65R17 or an inch code like 33x12.50R17.

Speed difference between two tire sizes is proportional to the circumference change. Actual Speed = Indicated Speed × (New Circumference / Original Circumference). Since circumference = π × diameter, this simplifies to: Actual Speed = Indicated Speed × (New Diameter / Old Diameter).

Example: Going from 225/65R17 (28.5" diameter) to 235/65R17 (29.0" diameter): ratio = 29.0/28.5 = 1.018. At 60 mph indicated: actual = 60 × 1.018 = 61.1 mph. Our calculator shows the exact error at every common speed from 20–90 mph.

This calculator compares two tire sizes mathematically — it shows diameter difference, speedometer error, and whether the change stays within the ±3% OEM tolerance. However, it does not contain a vehicle fitment database and cannot tell you which specific tire sizes are approved for your make and model.

For fitment, check: (1) your driver's door jamb tire placard, (2) the owner's manual for approved sizes, (3) fender and suspension clearance measurements, and (4) a professional tire shop. A vehicle-specific fitment database is on our roadmap.

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Last updated Jun 14, 2026

Tire Size Calculator

Size Comparison

What This Means

Tire Dimensions

How to Use This Tire Size Calculator

What is a Tire Size Calculator?

How Tire Size is Calculated

Safe vs Unsafe Tire Size Changes

Tips for Changing Tire Size

Worked Examples

Can This Calculator Tell What Size Tires Fit My Car?

Frequently Asked Questions

Embed Tire Size Calculator

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