Will bigger tires affect my gas mileage?

Yes. Larger, wider tires often have higher rolling resistance and are heavier, which can decrease your fuel economy. However, a slightly taller tire (larger diameter) can effectively lower your gear ratio, potentially improving highway mileage at the cost of city acceleration.

Can I put different size tires on the front and back?

This is called a 'staggered' setup and is common on rear-wheel-drive sports cars. However, for most front-wheel-drive and all-wheel-drive (AWD) vehicles, you should keep all four tires the same size. On AWD cars, different diameters can destroy your center differential.

What is the 'Load Index'?

The Load Index is a number (e.g., 91, 95, 100) that indicates the maximum weight a tire can support. When changing tire sizes, never choose a tire with a lower load index than your factory tires. Doing so is dangerous and can lead to blowouts.

Do I need an alignment after new tires?

It is highly recommended. While changing tires doesn't directly change your alignment, an alignment ensures your new (and expensive) tires wear evenly. If you're changing wheel sizes significantly, an alignment is mandatory.

Tire Size Calculator & Comparison Tool - Speedometer Error Check

Written by Marko Šinko•Category: Vehicle Specs & Maintenance

Why Tire Size Comparison Matters

Changing your vehicle's tires is one of the most impactful modifications you can make. Whether you're upgrading for better performance, switching to winter tires for safety, or simply enhancing the aesthetic appeal of your car with larger rims, understandingtire size comparison is critical.

The Tire Calculator above is designed to take the guesswork out of this process. By comparing your original equipment (OEM) tire size with a potential new size, you can instantly see how the change will affect your vehicle's speedometer, ground clearance, and overall geometry. Ignoring these calculations can lead to serious issues, including transmission damage, brake failure, and inaccurate speedometer readings.

How to Read Your Tire Sidewall

Before you can use the calculator effectively, you need to understand what the numbers on your tire sidewall mean. Let's break down a common size: 225/50R17.

225 (Width): This is the width of the tire in millimeters, measured from sidewall to sidewall. A wider tire (e.g., 245) generally offers more grip but may increase fuel consumption due to higher rolling resistance.
50 (Aspect Ratio): This is the height of the sidewall expressed as a percentage of the width. In this case, the sidewall height is 50% of 225mm. Lower numbers (like 35 or 40) indicate "low profile" tires, which offer better cornering stability but a stiffer ride.
R (Construction): This stands for "Radial," which is the standard construction method for almost all modern passenger tires.
17 (Rim Diameter): This is the diameter of the wheel (rim) in inches that the tire is designed to fit. You cannot mount a 17-inch tire on an 18-inch rim.

The Math Behind Tire Dimensions

Our calculator performs several complex calculations instantly to give you the most accurate data possible. These formulas are based on industry standards used by tire manufacturers and automotive engineers worldwide. If you're curious about the math, here's how we determine the diameter and circumference:

1. Calculating Sidewall Height

First, we convert the width and aspect ratio into a physical height in inches:
Sidewall (in) = (Width (mm) × (Aspect Ratio / 100)) / 25.4

2. Calculating Total Diameter

The total diameter of the tire is the rim diameter plus two sidewalls (top and bottom):
Diameter = Rim Diameter + (2 × Sidewall)

3. Calculating Circumference

The circumference determines how far the tire travels in one revolution. This is crucial for speedometer accuracy:
Circumference = Diameter × π (3.14159)

Speedometer Error: The Hidden Danger

Your car's speedometer doesn't actually measure speed directly; it measures how fast your transmission is spinning. It is calibrated based on the circumference of your factory-installed tires.

If you switch to a larger diameter tire, it travels further with each revolution. This means your car is moving faster than the speedometer thinks. For example, if you increase your tire diameter by 5%, when your speedometer reads 60 mph, you might actually be traveling at 63 mph. This can lead to unintentional speeding tickets.

Conversely, a smaller diameter tire will spin faster to cover the same distance, causing your speedometer to read higher than your actual speed. This will also cause your odometer to rack up miles faster than reality, potentially hurting your vehicle's resale value.

The 3% Rule

Industry experts and manufacturers generally recommend staying within 3% of the original tire diameter. Exceeding this limit can cause:

Brake Failure: ABS systems rely on precise wheel speed data. Drastic size changes can confuse the computer.
Transmission Issues: Modern automatic transmissions shift based on calculated vehicle speed. Incorrect data can cause erratic shifting.
Rubbing: Tires that are too tall or wide may rub against the suspension components or wheel well liners, especially during turns or over bumps.

Plus Sizing: Upgrading Your Wheels

"Plus Sizing" is the practice of increasing your wheel diameter while decreasing the tire's aspect ratio to maintain the same overall diameter. This allows you to run larger rims without messing up your speedometer or gear ratio.

Plus 1: Increase rim size by 1 inch (e.g., 16" to 17") and decrease aspect ratio (e.g., 55 to 45).
Plus 2: Increase rim size by 2 inches (e.g., 16" to 18") and further decrease aspect ratio (e.g., 55 to 40).

Use our calculator to experiment with different combinations. If you increase the rim size, you must decrease the aspect ratio to keep the diameter consistent.

Frequently Asked Questions

External Resources

For more detailed information on tire safety and regulations, visit the NHTSA Tire Safety page. If you're looking for specific tire reviews, Tire Rack's Tech Center is an excellent resource.

Tire Size Calculator & Comparison Tool