Tire Size Calculator
Calculate tire dimensions and compare different sizes
How to Use This Tire Size Calculator
- Find your current tire size on the sidewall (format: 225/45R17)
- Enter the width (225), aspect ratio (45), and rim diameter (17) for your original tire
- Enter the same values for the new tire size you're considering
- Click 'Compare Tire Sizes' to see diameter, speedometer error, and clearance differences
Example: Upgrading from 225/45R17 to 235/40R18: new tire is 0.38 inches smaller in diameter, your speedometer will read 0.9 mph fast at 60 mph, and you'll have slightly less sidewall cushion.
Tip: Stay within 3% of your original tire diameter to avoid significant speedometer errors and maintain proper vehicle handling.
Why Use a Tire Size Calculator?
Changing tire sizes affects your speedometer accuracy, fuel economy, ride comfort, and whether tires will even fit without rubbing.
- Verify new tires will fit before purchasing (clearance check)
- Calculate speedometer error when upsizing wheels or tires
- Compare plus-sizing options (bigger wheels with lower profile tires)
- Determine actual speed when speedometer shows 60 mph
- Check if winter tire size will maintain similar diameter
- Calculate odometer accuracy for lease mileage tracking
Understanding Your Results
Focus on overall diameter change to understand the practical impact on your vehicle.
| Result | Meaning | Action |
|---|---|---|
| 0-1% diameter change | Equivalent size | Safe swap with negligible speedometer impact |
| 1-3% diameter change | Acceptable variance | Most vehicles handle this well; speedometer will be slightly off |
| 3-5% diameter change | Noticeable difference | Check clearance carefully; may affect ABS, traction control |
| 5%+ diameter change | Significant change | Requires speedometer recalibration; may void warranty |
Meaning: Equivalent size
Action: Safe swap with negligible speedometer impact
Meaning: Acceptable variance
Action: Most vehicles handle this well; speedometer will be slightly off
Meaning: Noticeable difference
Action: Check clearance carefully; may affect ABS, traction control
Meaning: Significant change
Action: Requires speedometer recalibration; may void warranty
Note: Larger diameter tires make your speedometer read SLOWER than actual speed. Smaller tires make it read FASTER.
About Tire Size Calculator
Formula
Overall Diameter = Rim Diameter + (2 × Sidewall Height) Sidewall Height = Width × (Aspect Ratio ÷ 100), converted from mm to inches by dividing by 25.4. The sidewall appears twice in the diameter because it exists on both top and bottom of the wheel.
Current Standards: Most manufacturers recommend staying within 3% of OEM tire diameter. Exceeding 3% may trigger warning lights, affect traction control algorithms, and potentially void powertrain warranty coverage.
Frequently Asked Questions
What do the numbers on my tire sidewall mean?
A tire marked 225/45R17 means: 225mm tread width, sidewall height is 45% of the width (101mm), R indicates radial construction, and 17 is the wheel diameter in inches. Additional markings include speed rating (H, V, W, Y) and load index (91, 94, etc.).
Will bigger tires affect my gas mileage?
Yes, but the effect varies. Larger diameter tires lower your engine RPM at highway speeds (better economy) but add rotating mass (worse economy). Wider tires increase rolling resistance and aerodynamic drag. Net effect is usually 1-3% worse fuel economy for significantly larger tires.
What is plus-sizing and is it safe?
Plus-sizing means increasing wheel diameter while decreasing tire sidewall height to maintain similar overall diameter. Plus-1 (1 inch larger wheel) or Plus-2 sizing is common and safe when done correctly. This improves handling and appearance but reduces ride comfort due to less sidewall cushion.
How do I know if larger tires will fit my car?
Check three clearances: fender well (top), suspension components (inside), and fender lip (outside). Turn wheels fully left and right while checking. Most vehicles have 5-10mm of clearance margin. Online forums for your specific vehicle often document maximum fitments other owners have tested.
Does tire size affect my speedometer and odometer?
Yes. A 3% larger tire means your actual speed is 3% higher than displayed—at an indicated 60 mph, you're really going 61.8 mph. Your odometer will also under-count miles by 3%, which matters for warranty claims and lease mileage. Some vehicles allow speedometer recalibration through the dealer.