See exactly how a tire change moves your speedometer.
Enter a tire size in the standard width, aspect ratio and wheel format. Get the overall diameter, circumference and revolutions per mile, then compare a new size to see the real speedometer and odometer error.
Original tire
Reads as 225/45R17.
Reads as 255/40R18.
What this tire change does
Original vs new diameter
Overall diameter, inches
Speedometer reading vs actual speed
Tire scorecard
Speed correction table
With the new tire fitted, what your speedometer shows versus how fast you are really going.
| Speedometer reads | Actual speed | Difference | Odometer per 100 mi |
|---|
Tire sizing, explained
Reading Tire Size Notation
A tire marked 225/45R17 has a width of 225mm, an aspect ratio of 45 (sidewall height is 45% of width), and fits a 17-inch wheel. The R indicates radial construction. Understanding this notation is essential for choosing replacement tires.
How Overall Diameter Is Calculated
The overall tire diameter equals the wheel diameter plus twice the sidewall height. Sidewall height is the tire width multiplied by the aspect ratio divided by 100, then converted from millimeters to inches (divide by 25.4).
Revolutions Per Mile
Revolutions per mile indicates how many times the tire rotates over one mile. It is calculated by dividing 63,360 (inches per mile) by the tire circumference in inches. This affects your speedometer accuracy and odometer readings.
Effects of Changing Tire Size
Changing tire size affects speedometer accuracy, ride quality, handling, and fuel economy. A larger diameter tire makes the speedometer read slower than actual speed. Keep the overall diameter within 3% of the original to avoid problems.
Common questions
How does tire size affect my speedometer?
Larger tires cover more ground per revolution, making the speedometer read lower than actual speed. For example, a 3% larger tire means your speedometer shows 60 mph when you are actually traveling 61.8 mph.
What does the aspect ratio mean?
The aspect ratio is the sidewall height expressed as a percentage of the tire width. A 225/45 tire has a sidewall height of 225 x 0.45 = 101.25mm. Lower aspect ratios mean shorter sidewalls, which improve handling but reduce ride comfort.
Can I put different size tires on my car?
You can change tire size within limits. Keep the overall diameter within 3% of the original to maintain speedometer accuracy and avoid clearance issues. Consult your vehicle manual or a tire professional for safe alternative sizes.
What are plus-sizing and minus-sizing?
Plus-sizing increases wheel diameter while decreasing the aspect ratio to maintain overall diameter. Minus-sizing does the opposite. For example, going from 16-inch to 17-inch wheels with a lower profile tire keeps the same overall diameter.
Estimates for planning only. Real fit depends on wheel width, offset, load rating and clearance. Keep the overall diameter within 3% of the factory size and confirm any change with a tire professional before you drive.