How to Calculate Tire Size — Diameter, Speedometer & Fitment
Introduction
Choosing the right tyre size is one of the most impactful modifications you can make to your vehicle—yet it’s also one where a small error can lead to big consequences. An incorrect size can cause your speedometer to misread by several miles per hour, trigger warning lights for your anti-lock brakes (ABS), or even result in dangerous rubbing against your wheel arch or suspension components. Learning how to calculate tyre size manually empowers you to compare dimensions accurately, predict fitment issues, and ensure your vehicle remains safe, legal, and reliable.
This comprehensive guide explains how to decode the alphanumeric code on your tyre’s sidewall (e.g., 225/45R17), convert it into real-world measurements like overall diameter and circumference, and use those values to assess speedometer error, gearing changes, and clearance. You’ll also learn how to compare different sizes, understand the critical 3% rule, and account for real-world variables like rim width and suspension geometry. Whether you’re upgrading for performance, aesthetics, or off-road capability, this knowledge is your first line of defence against costly mistakes.
Decoding the Tyre Sidewall: From Code to Dimensions
Every modern tyre carries a standardised code that precisely defines its dimensions. Understanding this code is the foundation of all calculations.
The Standard Tyre Code: P225/45R17
- P: Vehicle type (Passenger). "LT" = Light Truck.
- 225: Section width in millimetres—the width from sidewall to sidewall.
- 45: Aspect ratio—the height of the sidewall as a percentage of the section width.
- R: Radial construction (nearly universal today).
- 17: Rim diameter in inches—the wheel size the tyre is designed to fit.
Step-by-Step Calculation of Key Dimensions
1. Calculate Sidewall Height
Sidewall Height (mm) = Section Width × (Aspect Ratio / 100)
Example: For 225/45:
225 × 0.45 = 101.25 mm
2. Convert to Inches
Sidewall Height (in) = Sidewall Height (mm) / 25.4
101.25 / 25.4 ≈ 3.99 in
3. Calculate Overall Diameter
Overall Diameter (in) = (2 × Sidewall Height) + Rim Diameter
(2 × 3.99) + 17 = 24.98 in
4. Calculate Circumference
Circumference (in) = Diameter × π
24.98 × 3.1416 ≈ 78.48 in
5. Calculate Revolutions Per Mile
Revs/Mile = 63,360 (in/mile) / Circumference (in)
63,360 / 78.48 ≈ 807 revs/mile
Understanding the Impact of Size Changes
Speedometer and Odometer Error
Your vehicle’s computer calculates speed based on revolutions per mile. A larger tyre covers more ground per revolution, causing the speedometer to under-read.
Actual Speed = Speedometer Reading × (New Diameter / Original Diameter)
Example: If your speedo reads 60 mph with tyres 3% larger, your actual speed is 60 × 1.03 = 61.8 mph.
Gearing and Performance
Larger tyres effectively raise your gearing, which can:
- Reduce acceleration
- Lower engine RPM at highway speeds (improving fuel economy slightly)
- Increase strain on the drivetrain
Clearance and Fitment
- Overall diameter affects vertical clearance (fender gap)
- Section width affects lateral clearance (strut/suspension)
Both must be checked, especially when turning or under full suspension compression.
The 3% Rule and Advanced Fitment Strategies
The 3% Diameter Rule
As a general guideline, keep your new tyre’s overall diameter within ±3% of the original. This ensures:
- Speedometer accuracy within ~2 mph at 70 mph
- Proper functioning of ABS, traction control, and stability systems
- Minimal impact on gearing and powertrain calibration
Rim Width Compatibility
Every tyre has a recommended rim width range. Mounting outside this range can cause:
- Too narrow: Bulging sidewalls, uneven wear, risk of de-beading
- Too wide: Stretched sidewalls, harsh ride, increased vulnerability to damage
Consult the tyre manufacturer’s specifications before purchasing wheels.
Accounting for Real-World Variables
- Tyre brand/model: Actual dimensions can vary by ±0.5" even with identical codes
- Tread depth: Aggressive off-road tyres may measure 0.5–1" larger than street tyres
- Inflation pressure: Affects sidewall bulge and effective diameter
- Suspension modifications: Lifted or lowered vehicles have different clearance envelopes
The Importance of Wheel Offset
Offset (ET) determines how far the wheel sits in or out relative to the hub. Even with the correct tyre size, an incorrect offset can cause:
- Inner rubbing on struts or brake components
- Outer rubbing on fender lips or liners
Always model tyre + wheel together—not just the tyre.
Worked Examples & Scenario Planning
Example 1: OEM+ Upgrade (Safe Change)
Original: 205/55R16
New: 225/45R17
Calculations:
- Original diameter:
2 × (205 × 0.55 / 25.4) + 16 = 24.88 in - New diameter:
2 × (225 × 0.45 / 25.4) + 17 = 24.98 in - Difference:
(24.98 - 24.88) / 24.88 = 0.4%
Verdict: Well within 3%—safe for speedometer and electronics.
Example 2: Off-Road Upgrade (Major Change)
Original: 255/75R17 (~32.1" diameter)
New: 35x12.50R18 (35" diameter)
Calculations:
- Difference:
(35 - 32.1) / 32.1 = 9.0%
Verdict: Requires speedometer recalibration, likely a lift kit, and may trigger ABS errors.
Example 3: Common Mistake (Unsafe Change)
Original: 205/50R16 (24.07")
New: 215/60R15 (25.16")
Calculations:
- Difference:
+4.5%
Verdict: Outside 3%—expect speedometer error and potential rubbing.
Practice Problems (Try These!)
- Your car has
225/45R18tyres (26.0"). You find245/40R19(26.7"). Is this acceptable? - Calculate the speedometer error if you install tyres that are 2.5% larger.
- Why might a 235/45R18 tyre from Brand A measure differently than from Brand B?
Answers:
(26.7 - 26.0) / 26.0 = 2.7%→ Acceptable (but check width/offset)- At 70 mph speedo reading:
70 × 1.025 = 71.75 mph→ +1.75 mph error - Manufacturing tolerances, tread pattern, and casing design cause slight variations.
How do I read a tyre size like 35x12.50R18?
This is flotation sizing, common for off-road tyres:
- 35: Overall diameter in inches
- 12.50: Section width in inches
- R18: Radial construction on 18-inch rim
To compare with metric sizes, convert to metric or calculate diameter directly.
What is the aspect ratio, and why does it matter?
The aspect ratio is the sidewall height as a percentage of width. A lower ratio (e.g., 35) means a shorter, stiffer sidewall—better for handling but harsher ride. Higher ratios (e.g., 65) offer more cushioning and comfort.
Can I use a tyre size calculator for winter tyres?
Yes—but note that winter tyres often have slightly narrower section widths and taller sidewalls for better snow traction. Always verify fitment, as the overall diameter should still match within 3%.
Do I need to recalibrate my speedometer after changing tyres?
If the new diameter is more than 3% different, yes. Many modern vehicles can be recalibrated using an OBD2 tool or dealership scan tool. Some aftermarket systems (e.g., Superchips) offer recalibration modules.
How does tyre size affect fuel economy?
Larger, heavier tyres increase rolling resistance and unsprung weight, typically reducing fuel economy by 1–3%. However, taller gearing from larger tyres can partially offset this at highway speeds.
What is Plus Sizing?
Plus sizing means increasing the wheel diameter while decreasing the aspect ratio to maintain the same overall diameter. Example: 205/55R16 → 225/45R17. This improves handling and aesthetics without affecting speedometer accuracy.
Can I mix different tyre sizes on my car?
Generally, no—especially on the same axle. Some performance cars use staggered setups (wider rear tyres), but this is engineered into the vehicle. Mixing sizes arbitrarily can cause handling instability and damage drivetrain components (especially in AWD vehicles).
How do I check for rubbing before buying?
- Use a tyre size calculator to compare diameters and widths
- Check owner forums for your specific vehicle
- Ask your tyre shop to test-fit one wheel/tyre
- Perform a full-lock turn and simulate suspension compression (e.g., with a jack)
What are load index and speed rating, and do they matter?
- Load Index: Maximum weight the tyre can support (e.g., 91 = 615 kg)
- Speed Rating: Maximum speed (e.g., V = 240 km/h)
Your new tyres must meet or exceed the original equipment (OE) specifications for safety and legality.