Plus 1 Tire Size Calculator: How It Works

A plus 1 tire size calculator helps you find the correct tire size when you increase your wheel diameter by one inch (for example, from 16-inch to 17-inch wheels). You enter your current tire size and the new wheel diameter, and the calculator returns the new tire width and aspect ratio that keeps the overall diameter nearly the same. This is the key to avoiding speedometer errors and clearance issues.

Formula

The overall diameter of a tire is the wheel diameter plus twice the sidewall height. The sidewall height is the tire width (in millimeters) multiplied by the aspect ratio (as a percentage), then converted to inches. The formula is:

Overall diameter (in) = (Width × Aspect Ratio) / 1270 + Wheel Diameter (in)

Where:

• Width – tire width in millimeters (e.g., 205)
• Aspect Ratio – sidewall height as a percentage of width (e.g., 55 means 55%)
• Wheel Diameter – rim diameter in inches (e.g., 16)
• 1270 – conversion factor: 1 inch = 25.4 mm, and the aspect ratio is a percentage, so 2 × 25.4 × 100 = 5080, but we divide by 2? Actually the standard formula is: (Width × Aspect Ratio) / 2540 × 2? Let’s be precise: Sidewall height in inches = (Width × (Aspect Ratio/100)) / 25.4. Then overall diameter = Wheel Diameter + 2 × Sidewall height. That simplifies to: Overall diameter = Wheel Diameter + (Width × Aspect Ratio) / 1270. Yes, because 2 / (25.4 × 100) = 2/2540 = 1/1270.

Worked Example

Using the default values: current tire 205/55R16, new wheel diameter 17 inches.

1. Current sidewall height: (205 × 55) / 1270 = 11275 / 1270 = 8.878 inches
2. Current overall diameter: 8.878 + 16 = 24.878 inches
3. New overall diameter (same tire width and aspect ratio): 8.878 + 17 = 25.878 inches
4. Difference: (25.878 – 24.878) / 24.878 × 100 = 4.02% larger

So a 205/55R17 would be about 4% larger, which is too much. To keep the diameter close, you need a lower aspect ratio, typically 50 or 45. The calculator helps you find the right combination.

What Does “Plus 1� Mean?

“Plus 1� (or “+1�) is a common upsizing strategy where you increase the wheel diameter by one inch while reducing the tire’s aspect ratio to maintain the same overall rolling circumference. For example, going from 16-inch wheels to 17-inch wheels is a plus 1 size. The goal is to keep the overall diameter within about 1-2% of the original to preserve speedometer accuracy, ride height, and clearance.

Why do people do this? Larger wheels often improve handling and appearance, but they also make the ride stiffer and increase tire cost. The plus sizing system is used by tire shops and enthusiasts to choose a compatible tire size.

How to Use the Calculator

To use the calculator, you need to know your current tire size (printed on the sidewall, like P205/55R16) and the new wheel diameter you want. Enter the width, aspect ratio, and current wheel diameter, then the new wheel diameter. The calculator shows the current and new overall diameters and the percentage difference. If the difference is more than about 2%, you should adjust the aspect ratio (or width) to bring it back within range.

For a plus 1 size, you typically reduce the aspect ratio by about 5 to 10 points. For example, a 205/55R16 becomes a 205/50R17 or 225/45R17. The exact size depends on availability and recommended fitments.

Common Mistakes and Misconceptions

• Ignoring width changes: If you increase tire width, the sidewall height changes even if the aspect ratio stays the same. Always recalculate.
• Forgetting the speedometer error: A larger diameter makes the speedometer read slower than actual speed. A 3% larger tire means at 60 mph you are actually going 61.8 mph.
• Assuming plus sizing is always 1 inch: Plus 2 and plus 3 are also common, but the same principle applies.
• Not checking clearance: Even if the diameter is correct, a wider tire may rub on the fender or suspension.

Choosing the Right Plus 1 Tire Size

To choose a plus 1 tire size, start with the calculator to find a combination that keeps the diameter within 1-2% of the original. Then consider the following:

• Width: You can keep the same width or go wider, but wider tires may require a wider wheel.
• Aspect ratio: Lower it to compensate for the larger wheel. The calculator helps you find the exact number.
• Load rating and speed rating: The new tire must meet or exceed the vehicle’s requirements. Check the owner’s manual.
• Wheel width: The new wheel must be within the recommended range for the tire width.

For example, if your original is 225/45R17, a plus 1 would be 18-inch wheels. Using the calculator, 225/40R18 gives a diameter difference of about -0.5%, which is ideal.

Safety and Accuracy Considerations

Keeping the overall diameter close to stock is crucial for safety. A significant difference can cause:

• Speedometer and odometer inaccuracies
• ABS and traction control malfunctions
• Transmission shift points being off
• Increased stress on brakes and suspension
• Potential rubbing on fenders or brake components

Always verify that the new tire and wheel combination clears the brake calipers and suspension at full lock and full compression. If you are unsure, consult a tire professional.

Frequently Asked Questions

What is the difference between plus 1 and plus 2 sizing?

Plus 1 increases the wheel diameter by 1 inch, plus 2 by 2 inches. Each step requires a corresponding decrease in tire aspect ratio to keep the overall diameter similar. For example, a 205/55R16 plus 1 might be 205/50R17, and plus 2 might be 205/45R18.

Can I use a plus 1 tire size without changing the wheels?

No. Plus 1 specifically means increasing the wheel diameter. You cannot fit a 17-inch tire on a 16-inch wheel. If you only want to change tire width or aspect ratio on the same wheels, that is a different modification.

How much can the tire diameter differ before it’s a problem?

Most manufacturers recommend staying within 1-2% of the original overall diameter. A difference larger than 3% can cause noticeable speedometer errors and may affect vehicle systems. The calculator shows the percentage difference, so aim for less than 2%.

Does a plus 1 size affect fuel economy?

Yes, often slightly. Larger wheels are heavier, and lower-profile tires have more rolling resistance. You may see a small decrease in fuel economy, typically 1-2 mpg, depending on the vehicle and driving conditions.

Will a plus 1 size improve handling?

Generally, yes. Lower-profile tires have less sidewall flex, which can improve cornering response and steering feel. However, the ride becomes harsher, and there is a higher risk of wheel damage from potholes.

Do I need to recalibrate my speedometer after plus sizing?

If the overall diameter changes by more than about 2%, the speedometer will be inaccurate. Many modern vehicles can be recalibrated with a diagnostic tool, but it’s best to choose a size that keeps the diameter within 1% to avoid this.

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in w | Current tire width (mm) | mm | 205 | 0
in a | Current aspect ratio (%) | % | 55 | 0
in r | Current wheel diameter (in) | in | 16 | 0
in nr | New wheel diameter (in) | in | 17 | 0
out dia | Current overall diameter (in) | in | (w * a / 1270) + r | 2
out ndia | New overall diameter (in) | in | (w * a / 1270) + nr | 2
out diff | Diameter difference (%) | % | ((ndia - dia) / dia) * 100 | 2