Tire Size Calculator

Use this tire size calculator to compare two tire sizes, convert metric dimensions to inches, and see how a tire swap affects your speedometer. Enter your current and desired tire sizes above, and the calculator returns diameter, width, sidewall height, circumference, and revolutions per mile for each option.

This tool gives you a quick visual and numerical comparison so you can decide whether a new tire fits your vehicle before you buy.

Tire Size Comparison Calculator

The calculator above accepts standard metric tire sizes (like 225/65R17) and returns a side-by-side breakdown of every key dimension. Enter two tire sizes to compare diameter, section width, sidewall height, circumference, and speedometer difference.

You can also use it as a tire size calculator to find which alternate sizes are close enough to your original equipment (OE) spec. A general guideline is to stay within about 3% of the original tire diameter. Going beyond that range can affect speedometer accuracy, ABS performance, and whether the tire clears your fenders and suspension.

How to Calculate Tire Diameter and Section Width

A standard metric tire size like 235/75R15 contains three numbers. Each one tells you something different about the tire's physical dimensions.

• 235 is the section width in millimeters, measured from sidewall to sidewall at the widest point.
• 75 is the aspect ratio, which tells you the sidewall height as a percentage of the width.
• R15 means the tire fits a 15-inch rim (the "R" stands for radial construction).

To calculate the overall tire diameter, you combine the wheel diameter with the height of both sidewalls. The formula is straightforward.

Rim and Wheel Size

The last number in a metric tire size is the wheel diameter in inches. A tire marked R17 mounts on a 17-inch rim. This number sets the baseline for the overall height calculation.

When you want to change to a new wheel size, the tire's sidewall height must adjust to keep the overall diameter similar. Moving to a larger wheel usually means choosing a tire with a lower aspect ratio. Moving to a smaller wheel means a taller sidewall.

For example, switching from a 225/65R17 to a 225/60R18 changes the wheel size but keeps the overall diameter close. The calculator handles this comparison automatically.

Aspect Ratio and Sidewall Height

The second number on a metric tire is the aspect ratio. It tells you that the sidewall height is a percentage of the section width.

For a 235/75R15 tire:

1. Section width = 235 mm
2. Aspect ratio = 75%, so sidewall height = 235 × 0.75 = 176.25 mm
3. Convert sidewall to inches: 176.25 ÷ 25.4 = 6.94 inches
4. Overall diameter = wheel diameter + (2 × sidewall height) = 15 + (2 × 6.94) = 28.88 inches

A higher aspect ratio means a taller sidewall, which adds cushion but increases the overall diameter. A lower aspect ratio gives a shorter, stiffer sidewall, common on sport and performance tires.

Compare Tire Sizes Side by Side

The most practical reason to compare tire sizes is to check whether a new tire fits without rubbing and how much it changes your speedometer reading. The calculator displays both tire specs in parallel columns so differences are obvious at a glance.

Tire Diameter Comparison

Overall tire diameter (also called overall height) is the single most important number when comparing two tires. A taller tire covers more ground per revolution, which makes your speedometer read slower than your actual speed. A shorter tire does the opposite.

Stay within roughly 3% of the original diameter to keep speedometer error manageable and avoid clearance problems. The calculator shows the diameter difference in both inches and percentage so you can judge quickly.

Width Comparison Between Two Tires

Section width matters for fitment on the wheel and inside the wheel well. A tire that is significantly wider than stock may rub against suspension components or fender liners, especially during turns or over bumps.

Most wheels have a recommended tire width range. For example, an 8-inch-wide wheel typically accepts tires from about 215 mm to 245 mm wide. The calculator displays the width of the tire for each entry so you can spot potential clearance issues.

Keep in mind that width also affects traction, handling feel, and ride noise, though those effects are harder to quantify with a simple calculator.

Tire Size Conversion: MM to Inches

Metric tires use millimeters for width and a percentage for sidewall height. Inch measurement tires (common on some trucks and off-road vehicles) state the overall diameter and width directly, like 33x12.50R15.

To convert a metric tire size to inches:

1. Calculate sidewall height: width (mm) × aspect ratio ÷ 100
2. Convert sidewall to inches: divide by 25.4
3. Overall diameter = rim size + (2 × sidewall in inches)
4. Convert section width to inches: width (mm) ÷ 25.4

Example: A 285/70R17 converts to roughly 32.7 inches tall and 11.2 inches wide.

This conversion is useful when comparing metric tires to inch measurement tires, or when shopping for cheap truck tires and jeep tires listed in different formats.

Speedometer and Actual Speed With Different Tire Sizes

Your speedometer is calibrated to the original tire circumference. When you install tires with a different diameter, the number of revolutions per mile changes, and the speedometer reading drifts from your actual speed.

• Larger tires = fewer revolutions per mile = speedometer reads lower than actual speed (you are going faster than displayed).
• Smaller tires = more revolutions per mile = speedometer reads higher than actual speed (you are going slower than displayed).

The calculator estimates the speedometer error as a percentage and shows your actual speed at common speedometer readings (like 60 mph). This helps you understand real-world impact before you commit to new tires.

A 3% diameter change produces roughly a 3% speedometer error. At 60 mph indicated, that is about 1.8 mph off. Some vehicles allow speedometer recalibration through the dealer or an aftermarket tuner.

Tire Calculator for Trucks, SUVs, and Other Vehicles

The same math applies whether you drive a passenger car, truck, SUV, or Jeep. The difference is the range of sizes and the available clearance.

• Trucks and SUVs often have room for larger tires, especially with a lift kit. Common upgrades go from 31-inch to 33-inch or 35-inch tires.
• Passenger cars typically have tighter wheel wells, so you are limited to smaller diameter and width changes.
• Jeeps and off-road vehicles frequently run oversized tires that require fender trimming, suspension modifications, or both.

Use the calculator to compare your current spec against the new size you are considering. Pay attention to diameter difference, width, and potential speedometer error. If you plan to convert to a different wheel size at the same time, enter the new wheel diameter and adjust the aspect ratio to keep overall height close to stock.

Larger tires can also change your effective gear ratio, which affects acceleration and fuel economy. The calculator estimates revolutions per mile, which gives you a starting point for understanding that tradeoff.

What Tire Sizes Fit Your Wheel Size

Every wheel has a recommended range of tire widths that mount safely. Going outside that range can cause poor bead seating, sidewall bulge, or handling problems.

General guidelines for common wheel widths:

These are approximate ranges. Always verify with the tire manufacturer's fitment data for the specific tire model. Offset and backspacing also affect whether a wider tire clears the suspension and fender.

When you enter a tire size in the calculator, check the width against your wheel to make sure the combination is compatible.

Tire Comparison Estimates and Limitations

This calculator provides estimates based on standard tire dimension formulas. Keep a few things in mind.

• Actual tire dimensions can vary slightly by manufacturer and model, even within the same labeled size. A 245/70R16 from one brand may measure a few millimeters differently than the same size from another.
• The calculator does not account for tire pressure, load, or wear. A worn tire has a smaller effective diameter than a new one.
• Speedometer error estimates assume a perfectly calibrated speedometer at the original tire size. Some vehicles have slight factory calibration offsets.
• Clearance depends on your specific vehicle, suspension setup, wheel offset, and whether you have added a lift or leveling kit. The calculator cannot confirm physical fitment.

Use these results as a planning aid. Confirm fitment with a tire shop or your vehicle's documentation before purchasing. This tool estimates dimensions and differences. It does not replace professional guidance on safety, load rating, or compatibility.