Tire Size Calculator
Compare two tire sizes and see how they affect diameter, circumference and speedometer readings.
Enter the width, aspect ratio and wheel diameter for your current tire and your new tire.
Current Tire
New Tire
Tire Comparison
Enter both tire sizes above to compare them.
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About the Creator
MyCruncher — Founded by Bobby Dues
MyCruncher provides free online calculators designed to make everyday calculations easier to understand and use.
Meet Bobby Dues →Free Tire Size Calculator – Compare Tire Diameter, Circumference and Speedometer Difference
Thinking about changing the tire size on your car, truck or SUV? The free MyCruncher Tire Size Calculator helps you compare two tire sizes and understand how changing tire dimensions may affect your vehicle.
Enter the current tire width, aspect ratio and wheel diameter. Then enter the same measurements for the new tire size you are considering.
The calculator estimates overall tire diameter, circumference, revolutions per mile, percentage difference and speedometer-related changes.
Whether you're comparing replacement tires, researching a different wheel setup or planning a tire upgrade, this calculator provides helpful measurements before purchasing.
How to Use the Tire Size Calculator
The MyCruncher Tire Size Calculator compares your current tire with a proposed replacement tire.
- Enter Current Tire Width. Use the tire's section width in millimeters, such as 225.
- Enter Current Aspect Ratio. Enter the sidewall aspect ratio percentage, such as 60.
- Enter Current Wheel Diameter. Use the wheel diameter in inches, such as 16.
- Enter New Tire Width. Provide the width of the tire you want to compare.
- Enter New Aspect Ratio. Use the replacement tire's aspect ratio.
- Enter New Wheel Diameter. Enter the wheel diameter required by the proposed tire.
- Enter Speedometer Reading. Provide a reference speed such as 60 miles per hour.
- Click Compare Tire Sizes. Review the diameter, circumference and revolutions-per-mile results.
- Review Actual Speed. See the estimated speed difference based on the two nominal tire diameters.
How to Read Tire Size Numbers
A common passenger vehicle tire size is written in a format such as 225/60R16.
Each part of the tire size represents a different measurement or tire construction detail.
225 – Tire Width
The number 225 represents the nominal section width in millimeters.
Actual section width can vary with the manufacturer, tire model and measuring wheel width.
60 – Aspect Ratio
The number 60 means the nominal sidewall height is 60% of the section width.
R – Radial Construction
The letter R indicates radial tire construction. It does not mean radius.
16 – Wheel Diameter
The number 16 indicates that the tire fits a 16-inch nominal wheel diameter.
The replacement tire must match the wheel diameter on which it will be mounted.
How Is Tire Sidewall Height Calculated?
Sidewall height is found by multiplying the nominal tire width by the aspect ratio as a decimal.
225/60R16 Sidewall Example
For a tire with a width of 225 mm and an aspect ratio of 60:
To convert millimeters to inches, divide by 25.4.
This is the nominal height of one tire sidewall.
How to Calculate Overall Tire Diameter
The overall tire diameter includes the wheel diameter plus the sidewall height on both sides of the wheel.
Tire Diameter Example
For a 225/60R16 tire:
These are nominal geometric calculations, not actual loaded tire measurements.
Tire Size Comparison Example
Example: Suppose your current tire size is 225/60R16 and you are considering 235/60R16.
Example Tire Information
- Current Tire: 225/60R16
- New Tire: 235/60R16
- Speedometer Reading: 60 MPH
Step 1: Calculate Current Sidewall Height
Step 2: Calculate New Sidewall Height
Step 3: Calculate Current Tire Diameter
Step 4: Calculate New Tire Diameter
Step 5: Calculate Diameter Difference
Step 6: Calculate Percentage Difference
Step 7: Calculate Actual Speed Estimate
Assume the speedometer was originally calibrated to the current tire size.
The larger nominal tire travels farther per revolution. Actual speedometer performance can differ because of tire construction, wear, inflation, loading and vehicle calibration.
Tire Comparison Summary
| Measurement | Current | New |
|---|---|---|
| Size | 225/60R16 | 235/60R16 |
| Width | 225 mm | 235 mm |
| Sidewall | 135 mm | 141 mm |
| Diameter | 26.63 in | 27.10 in |
| Circumference | 83.66 in | 85.14 in |
| Revs/Mile | 757.35 | 744.15 |
The new tire's diameter is approximately 1.77% larger, and its nominal revolutions per mile are lower.
All example values use the mathematical dimensions of the tire-size labels. Real manufacturer measurements may differ.
How Do You Calculate Tire Circumference?
Tire circumference is the distance around the tire based on its estimated diameter.
Use the circle circumference formula.
For a 225/60R16 tire with an estimated diameter of 26.6299 inches:
A larger circumference means the tire travels farther during each theoretical revolution.
How Are Tire Revolutions Per Mile Calculated?
One statute mile contains 63,360 inches.
Divide the number of inches in a mile by tire circumference to estimate revolutions per mile.
Revolutions Per Mile Example
For a tire with an estimated circumference of 83.6604 inches:
This is a geometric estimate. Manufacturer-listed revolutions per mile can differ because a tire deforms while rolling under load.
How Does Tire Size Affect Speedometer Readings?
Speedometers are calibrated using vehicle-specific assumptions about wheel and tire rotation.
Changing the overall rolling circumference may affect the relationship between indicated speed and actual speed.
Larger Diameter Tires
With otherwise unchanged calibration, larger tires may cause the vehicle to travel faster than the speedometer indicates.
Smaller Diameter Tires
Smaller tires may cause actual vehicle speed to be lower than indicated speed.
The formula assumes the speedometer was accurate with the original tire and the rolling diameter ratio follows the nominal dimensions.
Speedometer Difference Chart
The following comparison assumes a 1.77% increase in effective rolling circumference.
| Indicated MPH | Estimated Actual MPH |
|---|---|
| 20 | 20.35 |
| 30 | 30.53 |
| 40 | 40.71 |
| 50 | 50.89 |
| 60 | 61.06 |
| 65 | 66.15 |
| 70 | 71.24 |
| 75 | 76.33 |
| 80 | 81.42 |
These are illustrative values. They do not verify the accuracy of a specific vehicle's speedometer.
How Does Tire Size Affect the Odometer?
An odometer uses vehicle rotation measurements and calibration to estimate distance traveled.
A change in effective tire rolling circumference can change the reported distance when the system is not recalibrated.
If new tires have a greater rolling circumference, the vehicle may travel farther per counted revolution.
Actual odometer behavior depends on vehicle design and calibration.
Does a Larger Tire Increase Ground Clearance?
A taller tire may raise an axle's position above the road.
For an idealized diameter comparison, the approximate increase in axle height is half the diameter difference.
For our example tires:
Actual vehicle clearance varies with tire deflection, inflation pressure, suspension design and loading.
How Does Tire Size Affect Vehicle Gearing?
A change in rolling circumference changes the distance the vehicle travels per wheel revolution.
Larger tires can reduce engine RPM at a given road speed when the transmission gear and other conditions remain unchanged.
Smaller tires can produce the opposite effect.
The actual effect depends on the vehicle's transmission, differential gearing, tire dimensions and driving conditions.
Does Tire Width Affect Overall Diameter?
Tire width can affect overall diameter when aspect ratio and wheel diameter remain unchanged.
This happens because sidewall height is calculated as a percentage of tire width.
For example, a 235/60R16 tire has a larger nominal sidewall than a 225/60R16 tire.
However, changing the aspect ratio at the same time can offset the diameter difference.
Tire Diameter Comparison Chart
The following examples all use a 16-inch wheel.
| Tire Size | Nominal Diameter |
|---|---|
| 205/55R16 | 24.88 in |
| 205/60R16 | 25.69 in |
| 215/60R16 | 26.16 in |
| 225/55R16 | 25.74 in |
| 225/60R16 | 26.63 in |
| 225/65R16 | 27.52 in |
| 235/60R16 | 27.10 in |
| 235/65R16 | 28.03 in |
| 245/60R16 | 27.57 in |
These figures describe nominal diameter only. They do not indicate that all listed sizes are interchangeable or suitable for any vehicle.
How Close Should Two Tire Sizes Be?
A small diameter difference may reduce some speedometer and gearing changes, but there is no universal percentage that guarantees safe fitment.
Vehicle manufacturers can have specific requirements for approved tire sizes, load ratings, wheel dimensions, drivetrain systems, electronic stability control and clearance.
Use the calculator to compare dimensions, then verify the proposed size against the vehicle manufacturer's guidance.
Can You Change Tire Size Without Changing Wheels?
Sometimes, if the new tire has the same nominal wheel diameter and is approved for the wheel's width and design.
Matching the wheel diameter alone does not guarantee compatibility.
The new tire must also be appropriate for wheel width, load requirements and vehicle clearance.
Tire Fitment and Safety Considerations
- Wheel diameter. The tire must match the wheel's nominal diameter.
- Wheel width. Verify the tire manufacturer's approved wheel-width range.
- Load index. Use a tire with an appropriate load rating for the vehicle.
- Speed rating. Check the vehicle's required speed capability.
- Brake clearance. Wheel fitment must maintain appropriate brake component clearance.
- Suspension clearance. Check tire clearance throughout steering and suspension travel.
- Body clearance. Avoid rubbing against fenders, liners and other vehicle components.
- AWD and 4WD requirements. Some drivetrains require closely matched rolling circumferences across tires.
- Inflation and TPMS. Follow vehicle and tire manufacturer guidance.
- Speedometer calibration. Determine whether recalibration is necessary after a size change.
Common Tire Size Comparison Mistakes
- Confusing width with diameter. Tire width is not the overall tire diameter.
- Misreading aspect ratio. The ratio is a percentage of section width.
- Ignoring the wheel size. The tire must fit the wheel diameter.
- Comparing only tire width. Aspect ratio and wheel diameter also matter.
- Assuming nominal sizes equal actual dimensions. Real tire measurements vary by model.
- Ignoring load and speed ratings. These are important safety requirements.
- Assuming small diameter changes guarantee fitment. Clearance and vehicle requirements still need verification.
- Ignoring all-wheel-drive requirements. Mismatched tires can affect some AWD drivetrains.
Frequently Asked Questions
How do I compare two tire sizes?
Enter the width, aspect ratio and wheel diameter for each tire, then click Compare Tire Sizes.
What does 225/60R16 mean?
It describes a 225 mm nominal tire width, 60% aspect ratio, radial construction and 16-inch wheel.
How do I calculate tire diameter?
Convert the sidewall height into inches, double it, and add the wheel diameter.
How do I calculate tire circumference?
Multiply estimated tire diameter by π.
How many revolutions per mile does a tire make?
Divide 63,360 inches by the tire's estimated circumference in inches.
What is the diameter of a 225/60R16 tire?
Its nominal calculated diameter is approximately 26.63 inches.
What is the diameter of a 235/60R16 tire?
Its nominal calculated diameter is approximately 27.10 inches.
How much larger is 235/60R16 than 225/60R16?
The nominal diameter is approximately 1.77% larger.
Do larger tires affect speedometer readings?
Yes, when the effective rolling circumference changes without corresponding calibration.
Will bigger tires make my speedometer read slower?
Under unchanged calibration, a larger rolling circumference can make actual speed greater than indicated speed.
Will smaller tires affect the speedometer?
Yes. Smaller effective rolling circumference can make actual speed lower than indicated speed.
Does tire size affect the odometer?
It can change the relationship between measured wheel rotation and actual distance.
Does tire size affect RPM?
Yes. A change in effective rolling circumference can affect engine RPM at a given road speed.
Do larger tires increase ground clearance?
They may increase axle clearance by approximately half the diameter increase, before considering tire deflection and suspension factors.
Can I put a wider tire on the same wheel?
Possibly, if the tire manufacturer approves the wheel width and the vehicle has suitable clearance.
Is a 3% tire diameter difference always safe?
No. A percentage difference alone cannot confirm vehicle compatibility.
Can I use different tire sizes on an AWD vehicle?
Follow the vehicle manufacturer's requirements. Some AWD systems are sensitive to tire rolling circumference differences.
Does this calculator determine tire load rating?
No. It compares geometric dimensions and speedometer-related estimates.
Does it check actual vehicle fitment?
No. You must verify wheel, tire and vehicle compatibility separately.
Can I compare truck and SUV tires?
Yes, when the tires use compatible width, aspect ratio and wheel diameter inputs.
Does the calculator use actual manufacturer measurements?
No. It estimates dimensions from the tire size values you enter.
Does the calculator include tire wear?
No. Actual tread wear can change effective rolling diameter.
Is the MyCruncher Tire Size Calculator free?
Yes! The Tire Size Calculator is free for general tire dimension comparison.
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Important: The MyCruncher Tire Size Calculator provides geometric estimates for education and general planning. It does not confirm physical tire fitment, manufacturer approval, speedometer calibration, vehicle safety, load capacity or drivetrain compatibility. Actual tire dimensions, rolling circumference and revolutions per mile can differ from calculated nominal values. Before changing tire sizes, verify the vehicle manufacturer's requirements and tire specifications, and consult a qualified tire professional when needed.