Tire Size Calculator

Introduction

The Tire Size Calculator helps you understand what your tire numbers mean and compare different tire sizes for your car. Tire sizes can be confusing — they use a mix of millimeters, percentages, and inches all in one code printed on the side of your tire. This tool breaks those numbers down so you can see the actual width, sidewall height, and overall diameter of any tire. Whether you are shopping for new tires, thinking about switching to a bigger or smaller size, or just curious about what fits your vehicle, this calculator makes it simple. Just enter your tire size, and it does the math for you in seconds.

How to Use Our Tire Size Calculator

Enter your tire measurements to get full tire specifications, compare two tire sizes side by side, or browse a reference chart of common tire sizes. The calculator gives you overall diameter, sidewall height, circumference, revolutions per mile, speedometer correction, and a visual tire profile.

Vehicle Selection (Optional): If you don't know your tire size, pick your vehicle's year, make, model, and trim from the dropdown menus. The calculator will fill in your tire size for you automatically.

Format Toggle (Metric or Inch): Choose between metric format (like 265/70R17) or inch format (like 31.6x10.4R17) depending on how your tire size is listed.

Section Width: Enter the width of your tire in millimeters (metric mode) or inches (inch mode). This is the first number in a standard tire size. For example, in 265/70R17 the width is 265 mm.

Aspect Ratio: Enter the aspect ratio of your tire, which is the sidewall height shown as a percentage of the tire's width. In 265/70R17, the aspect ratio is 70. This field appears only in metric mode. You can use our Percentage Calculator if you need help understanding how percentages work in tire sizing.

Overall Diameter (Inch Mode Only): Enter the total height of the tire from top to bottom in inches. This replaces the aspect ratio when you use inch format.

Rim Diameter: Enter the diameter of your wheel rim in inches. This is the last number in a tire size. In 265/70R17, the rim size is 17 inches.

Tire Comparison Tab: Enter the section width, aspect ratio, and rim diameter for both your original tire (Tire 1) and your new tire (Tire 2). The calculator will show a visual size comparison, bar charts, a side-by-side specs table, and speedometer correction data so you can see exactly how the two sizes differ.

Reference Charts Tab: Use the rim size filter, diameter range filter, or search bar to browse a large list of common tire sizes along with their full specifications. This helps you find and compare tire sizes quickly without entering numbers one at a time.

Understanding Tire Sizes

Every tire has a set of numbers printed on its sidewall that tells you exactly how big it is. A typical tire size looks like 265/70R17. Each part of this code means something specific. The first number (265) is the section width — how wide the tire is in millimeters from one sidewall to the other. The second number (70) is the aspect ratio, which tells you the sidewall height as a percentage of the width. The "R" stands for radial construction, and the last number (17) is the rim diameter in inches. Together, these three numbers define the tire's overall size and shape.

How Tire Dimensions Are Calculated

To find the overall diameter of a tire, you need to combine the rim size with the sidewall height. The sidewall height is found by multiplying the section width by the aspect ratio. For example, a 265/70R17 tire has a sidewall height of 265 × 0.70 = 185.5 mm. Since there are two sidewalls (top and bottom), you add both to the rim diameter: (17 inches × 25.4 mm) + (2 × 185.5 mm) = 802.8 mm, or about 31.6 inches. From the overall diameter, you can then figure out the circumference (the distance the tire covers in one full rotation) and the revolutions per mile, which is how many times the tire spins to travel one mile. If you're curious about the relationship between circumference and diameter, the math is similar to concepts used in our Displacement Calculator.

Why Tire Size Matters

Changing your tire size affects more than just how your car looks. A taller tire covers more ground per rotation, which means your speedometer will read lower than your actual speed. A shorter tire does the opposite — your speedometer will show a higher number than you are really going. Most experts recommend staying within 3% of your original tire diameter to keep your speedometer accurate and avoid problems with your vehicle's safety systems like anti-lock brakes and traction control. You can use our Percent Change Calculator to quickly determine the percentage difference between your original and new tire diameters.

Wider tires give you more grip on the road because more rubber touches the ground. However, wider tires can also increase rolling resistance, which may lower your fuel economy slightly. Tires with a shorter sidewall (lower aspect ratio) tend to feel more responsive during steering but give a firmer ride. Tires with taller sidewalls absorb bumps better and are more comfortable on rough roads.

If you're also looking at window modifications alongside new tires, our Tint Calculator can help you figure out the right window tint levels for your vehicle. And if you're budgeting for a new set of tires as part of a vehicle purchase, the Auto Loan Calculator can help you plan your overall car financing.

Comparing Two Tire Sizes

When you are thinking about switching to a different tire size, it is important to compare the old and new sizes side by side. Key things to check include the difference in overall diameter, section width, and revolutions per mile. A bigger diameter means your car sits slightly higher and your odometer will under-count the miles you drive. A smaller diameter does the opposite. You should also make sure the new tire fits your wheel well and does not rub against fenders or suspension parts when turning or going over bumps. Similar to how a Percent Error Calculator measures deviation from a true value, checking the diameter difference percentage tells you how far off your speedometer will be with the new tires.

Metric vs. Inch Tire Sizing

Most passenger cars and trucks use the metric system for tire sizes (like 225/60R16). However, some off-road and light truck tires use inch-based sizing (like 31x10.5R15), where the first number is the overall diameter in inches and the second is the section width in inches. Both formats describe the same physical measurements — they just express them differently. You can convert between the two formats using simple math, or you can use the calculator above to handle the conversion automatically.

If you enjoy working on your vehicle and are planning other upgrades — like cycling as cross-training — our Bike Gear Ratio Calculator applies similar ratio concepts to bicycle drivetrain setups. The idea of gear ratios and wheel size affecting speed and distance is fundamentally the same whether you're on two wheels or four.

Frequently Asked Questions

What do the numbers on my tire mean?

A tire size like 265/70R17 has three parts. The first number (265) is the tire width in millimeters. The second number (70) is the aspect ratio, which is the sidewall height as a percent of the width. The last number (17) is the rim size in inches. The "R" just means radial, which is the type of tire construction.

How do I find my current tire size?

Look at the sidewall of your tire. You will see a code like 225/60R16 printed on it. You can also check the sticker on the inside of your driver's door jamb, or look in your vehicle's owner's manual. This calculator also lets you select your vehicle year, make, model, and trim to find your tire size automatically.

How much bigger can I go without causing problems?

Most experts say you should stay within 3% of your original tire diameter. Going beyond that can throw off your speedometer, odometer, and safety systems like ABS and traction control. Tires that are too big can also rub against your fenders or suspension when you turn.

Will changing tire size affect my speedometer?

Yes. A taller tire makes your speedometer read lower than your actual speed. A shorter tire makes it read higher. The speedometer correction section of this calculator shows you the exact difference at various speeds so you know how far off your reading will be.

What is sidewall height and why does it matter?

Sidewall height is the distance from the rim to the outer edge of the tire. It is calculated by multiplying the tire width by the aspect ratio. A taller sidewall gives a softer, more comfortable ride. A shorter sidewall gives sharper handling but a firmer ride. Sidewall height also affects your overall tire diameter.

What are revolutions per mile?

Revolutions per mile tells you how many times your tire spins to travel one mile. A smaller tire has more revolutions per mile because it covers less ground per spin. A larger tire has fewer revolutions per mile. This number affects your speedometer accuracy and odometer readings.

Can I put a different rim size on my car?

Yes, but you need to adjust the tire size to keep the overall diameter close to the original. This is called plus sizing. For example, if you go from a 16-inch rim to an 18-inch rim, you need a tire with a shorter sidewall so the total diameter stays about the same. Use the comparison tab to check this.

What is the difference between metric and inch tire sizes?

Metric sizes like 265/70R17 list width in millimeters, aspect ratio as a percentage, and rim size in inches. Inch sizes like 31.6x10.4R17 list overall diameter first, then width, both in inches, followed by rim size. They describe the same tire — just in different formats. This calculator handles both.

Does a wider tire give better traction?

Generally, yes. A wider tire puts more rubber on the road, which can improve grip during acceleration and cornering. However, wider tires can also increase rolling resistance, which may slightly reduce fuel economy. They can also be more prone to hydroplaning on wet roads.

How does tire size affect fuel economy?

Larger and wider tires are heavier and have more rolling resistance, which can lower your gas mileage slightly. Smaller, narrower tires tend to be more fuel-efficient. The difference is usually small — around 1 to 3 percent — but it adds up over time.

Will my odometer be wrong with different size tires?

Yes. Your odometer counts wheel rotations to measure distance. A larger tire covers more ground per rotation, so your odometer will under-count your actual miles. A smaller tire will make it over-count. Staying within 3% of your stock tire diameter keeps the error small.

What does the reference chart tab do?

The reference chart shows a large list of common tire sizes with all their specs already calculated. You can filter by rim size, diameter range, or search for a specific size. This is helpful when you want to browse options and find tires that match a certain diameter or width without entering each size one at a time.

How accurate is the speedometer correction in this calculator?

The speedometer correction is based on the mathematical difference between the tire circumferences. It is very accurate for showing the effect of tire size on speed readings. However, your actual speedometer may already have a small built-in error from the factory, so real-world results can vary slightly.

Can I use this calculator for truck tires?

Yes. This calculator works for cars, trucks, SUVs, and off-road vehicles. It supports tire widths from 105 mm to 405 mm and rim sizes from 13 to 30 inches, which covers most passenger and light truck tire sizes. You can also use inch format for off-road tire sizes.

What is aspect ratio?

Aspect ratio is the height of the sidewall as a percentage of the tire's width. An aspect ratio of 70 means the sidewall is 70% as tall as the tire is wide. A lower aspect ratio means a shorter, sportier sidewall. A higher aspect ratio means a taller, more cushioned sidewall.