Tire Size Comparison & Speedometer Error
Changing your wheels or tires? Crucial safety depends on keeping your new tire diameter close to the original factory specification. Our Tire Size Calculator compares two tire sizes visually and mathematically, showing you exactly how much your speedometer reading will change and whether the new setup is safe for your car.
Why Diameter Matters (The 3% Rule)
Most automotive engineers and serious retailers recommend keeping your new tire diameter within **3%** of the original tire. Exceeding this difference can cause:
Reading the Sidewall Numbers
Before using the calculator, find these three numbers on your tire sidewall:
| Number | Name | Meaning | Example |
|---|---|---|---|
| 1st (205) | Section Width | The width of the tire tread in millimeters. | 205mm wide |
| 2nd (55) | Aspect Ratio | The height of the sidewall as a % of width. | 55% of 205mm = 112.75mm |
| 3rd (16) | Rim Diameter | The diameter of the metal wheel in inches. | Fits 16-inch rim |
Plus Sizing (Upgrading Wheels)
A common modification is "Plus One" or "Plus Two" sizing—increasing rim size by 1 or 2 inches. To keep the total diameter the same, you must decrease the tire's aspect ratio.
**Example of a Perfect 'Plus One' Upgrade:**
Original: **205/55R16** (Diameter: 24.9")
New: **225/45R17** (Diameter: 25.0")
Result: Only **0.4% difference**. This is safe and will not significantly affect your speedometer.
**Example of a Perfect 'Plus One' Upgrade:**
Original: **205/55R16** (Diameter: 24.9")
New: **225/45R17** (Diameter: 25.0")
Result: Only **0.4% difference**. This is safe and will not significantly affect your speedometer.
Tire Size FAQ
Q:How much difference in tire size is acceptable?
Generally, a difference of less than 3% in overall diameter is considered safe for most vehicles. Staying within 1% is ideal. Differences larger than 3% can affect your ABS, traction control, and speedometer accuracy.
Q:Will bigger tires make my car faster?
Technically, taller tires increase your top speed potential (due to longer gearing) but reduce your acceleration (torque). However, your speedometer will read *slower* than you are actually going, which can lead to unintentional speeding tickets.
Q:What does 'R' mean in tire size?
'R' stands for Radial construction, which is the standard for almost all modern passenger car tires. It refers to how the internal plies are arranged (radially across the tire) rather than the rim size itself, though it sits right next to the rim number.
Q:Can I put wider tires on my stock rims?
Usually, yes, but only within limits. Each rim width supports a range of tire widths. For example, a 7-inch wide rim typically fits 195mm to 225mm tires. Exceeding recommended widths can lead to poor handling or tire failure.
Q:How does tire size affect gas mileage?
Wider tires generally increase rolling resistance and aerodynamic drag, slightly lowering MPG. Taller tires (larger diameter) effectively 'gear up' the car, which might improve highway MPG slightly but hurt city MPG due to the extra weight and inertia.