Understanding Tire Size Comparison
Changing your vehicle's tires is one of the most common modifications, but it's also one of the most misunderstood. Whether you're looking for a wider stance, better off-road performance, or simply replacing worn-out rubber with a different size, understanding the geometry is crucial. Our Tire Size Calculator allows you to visually compare two tire sizes and understand the impact on your speedometer, clearance, and gearing.
How to Read Tire Sizes
Before you can compare tires, you need to understand what the numbers on the sidewall mean. Let's break down a common size like 205/55R16:
- 205 (Width): The width of the tire in millimeters, measured from sidewall to sidewall.
- 55 (Aspect Ratio): The height of the sidewall as a percentage of the width. In this case, the sidewall height is 55% of 205mm.
- R (Construction): Stands for "Radial," the standard construction for modern passenger cars.
- 16 (Rim Diameter): The diameter of the wheel (rim) in inches that the tire is designed to fit.
Why Tire Diameter Matters
The most critical factor when changing tire sizes is the overall diameter. Your vehicle's speedometer, odometer, traction control, and ABS systems are all calibrated based on the factory tire diameter.
If you install a tire that is significantly taller or shorter than stock, it throws off these calculations. For example, a taller tire will travel further with each revolution than a shorter tire. This means your speedometer will readslower than you are actually traveling, putting you at risk of speeding tickets.
The 3% Rule
A general rule of thumb in the automotive industry is to keep the new tire diameter within 3% of the original equipment (OE) tire.
- Less than 3% difference: Generally safe. Speedometer error is negligible, and suspension clearance is usually sufficient.
- More than 3% difference: Risk of brake failure (due to increased leverage), handling issues, and rubbing against the wheel well or suspension components.
Our calculator automatically highlights this difference. If you see a warning, consider choosing a size closer to stock or consulting a professional mechanic.
Speedometer Error Explained
Let's say you switch from a 25-inch tire to a 30-inch tire. Since the new tire is larger, it covers more ground per revolution. However, your car's computer doesn't know the tire changed; it still counts revolutions based on the old size.
The Result: When your speedometer says 60 MPH, you might actually be doing 72 MPH. This is dangerous not only for legal reasons but also for safety systems like stability control, which rely on accurate wheel speed data.
Plus Sizing: Plus-1, Plus-2
"Plus Sizing" is the practice of increasing the wheel diameter while decreasing the tire's aspect ratio to maintain the same overall diameter. This improves handling by stiffening the sidewall but can reduce ride comfort.
- Plus-1: Increase rim by 1 inch, decrease aspect ratio (e.g., 16" to 17").
- Plus-2: Increase rim by 2 inches, decrease aspect ratio further (e.g., 16" to 18").
Use this calculator to find the perfect aspect ratio when upgrading your rims to ensure your speedometer stays accurate.
Frequently Asked Questions
Related Tools
If you are upgrading your vehicle, you might also be interested in our Car Loan Calculator to see if those new wheels fit your budget, or the MPG Calculator to track your efficiency after the swap.