How to calculate new wheel and tire size

## Understanding Wheel and Tire Size Calculations

Whether you’re enhancing the aesthetics of your vehicle or upgrading its performance, understanding how to calculate wheel and tire sizes is crucial. Mismatched wheel and tire combinations can lead to safety issues, premature wear, and compromised handling.

### Measuring Wheel and Tire Size

Before performing any calculations, you must accurately measure the existing wheel and tire sizes:

#### Wheel Size

– Diameter: Measured in inches, it’s the distance from the top to the bottom of the wheel.
– Width: Measured in inches, it’s the distance from one lip to the opposite lip of the wheel.
– Offset: Measured in millimeters (mm), it determines how far the wheel’s mounting surface is from the centerline.
– Positive offset: The mounting surface is closer to the outside of the wheel.
– Negative offset: The mounting surface is closer to the inside of the wheel.

#### Tire Size

– Width: Measured in millimeters (mm), it represents the distance from sidewall to sidewall.
– Aspect ratio: Expressed as a percentage, it indicates the height of the tire’s sidewall relative to its width.
– Diameter: Measured in inches, it’s the distance from one sidewall to the opposite sidewall, including the tread depth.

### Calculating New Wheel and Tire Size

Step 1: Determine the Desired Overall Diameter

The overall diameter of your new wheel and tire combination is crucial for maintaining speedometer accuracy, proper ride height, and suspension performance. To calculate the desired overall diameter:

– Find the original overall diameter by measuring the tire diameter and adding twice the wheel diameter.
– Consult your vehicle’s specifications or a reputable tire retailer to identify the recommended overall diameter.

Step 2: Adjust for Tire Aspect Ratio and Sidewall Height

– Larger aspect ratio: Results in a taller sidewall, which increases the overall diameter.
– Smaller aspect ratio: Produces a shorter sidewall, decreasing the overall diameter.

The formula for adjusting the aspect ratio is:

“`
New tire diameter = Original tire diameter x (New aspect ratio / Original aspect ratio)
“`

Step 3: Calculate New Wheel Diameter

– Smaller wheel diameter: Decreases the overall diameter.
– Larger wheel diameter: Increases the overall diameter.

The formula for adjusting the wheel diameter is:

“`
New wheel diameter = Original overall diameter – New tire diameter
“`

Step 4: Consider Offset and Wheel Width

– Positive offset: Pushes the wheel further inside the wheelbase, widening the stance.
– Negative offset: Positions the wheel further outside the wheelbase, narrowing the stance.

Wheel width can also affect the stance and overall diameter. A wider wheel will increase the overall diameter, while a narrower wheel will decrease it.

Step 5: Verify Clearance and Compatibility

Once you have calculated the new wheel and tire sizes, consult with a reputable tire retailer or automotive technician to ensure:

– Clearance between the tires and suspension components, wheelbase, and fenders.
– Compatibility with your vehicle’s braking, steering, and safety systems.

### Example Calculation

Suppose you have a vehicle with the following original wheel and tire specifications:

– Wheel diameter: 17 inches
– Tire width: 225 millimeters
– Tire aspect ratio: 55%
– Overall diameter: 28 inches

You want to upgrade to 18-inch wheels with a tire width of 235 millimeters and an aspect ratio of 45%.

Step 1: Determine the Desired Overall Diameter

Let’s assume the recommended overall diameter is 28 inches.

Step 2: Adjust for Tire Aspect Ratio

“`
New tire diameter = 28 inches x (45% / 55%) = 23.04 inches
“`

Step 3: Calculate New Wheel Diameter

“`
New wheel diameter = 28 inches – 23.04 inches = 4.96 inches
“`

So, the new wheel diameter is approximately 5 inches or 18 inches.

Step 4: Consider Offset and Wheel Width

Let’s assume you want to maintain the original offset and stance. Consult with a tire retailer for appropriate wheel width recommendations based on your vehicle specifications.

Step 5: Verify Clearance and Compatibility

Ensure the chosen wheel and tire combination provides sufficient clearance and is compatible with your vehicle’s systems.

### Important Considerations

– Speedometer accuracy: Ensure that the overall diameter of the new wheel and tire combination is within the vehicle’s recommended range to maintain accurate speedometer readings.
– Suspension geometry: Mismatched wheel and tire sizes can alter the suspension geometry, affecting handling, stability, and tire wear.
– Brake clearance: Larger wheels may require brake caliper modifications or spacers to prevent interference with the wheel spokes.
– Insurance implications: Unreported wheel and tire modifications may impact your insurance coverage in the event of an accident.

### Conclusion

Calculating new wheel and tire size is essential for maintaining proper vehicle performance and aesthetics. By following the steps outlined in this guide, you can confidently determine the ideal combination for your specific needs. However, it’s always advisable to consult with experienced professionals to verify compatibility and ensure the safety of your vehicle.