Tire Size Calculator & Diameter Comparison
Compare two tire sizes and instantly see the difference in overall diameter, sidewall height, circumference, revolutions per mile, ground clearance and approximate speedometer reading.
Use it when researching an alternative tire size, changing wheel diameter or simply trying to understand what numbers like 265/70R17 and 275/65R18 actually mean.
| Measurement | Original Tire | New Tire | Difference |
|---|---|---|---|
| Overall Diameter | — | — | — |
| Sidewall Height | — | — | — |
| Section Width | — | — | — |
| Circumference | — | — | — |
| Revolutions / Mile | — | — | — |
What Happens When the Speedometer Says 60 or 70 MPH?
If the vehicle was calibrated around the original tire diameter, changing tire diameter can change the relationship between indicated and actual road speed.
What Does 275/65R18 Mean?
The nominal section width is 275 millimeters.
The sidewall height is nominally 65% of the tire’s section width.
The tire is designed for an 18-inch wheel. “R” refers to radial construction.
How Tire Diameter Is Calculated
Metric tire sizes provide enough information to estimate their nominal dimensions mathematically.
width × (aspect ratio ÷ 100)
wheel diameter + (2 × sidewall height in inches)
overall diameter × π
Tire Diameter Affects More Than Appearance
A larger or smaller rolling diameter can change the relationship between indicated and actual vehicle speed.
A taller tire effectively lengthens gearing, while a shorter tire effectively shortens it.
Half of the total diameter increase approximately becomes additional axle height, assuming the tire actually measures as calculated.
Increasing width or diameter may affect fender, suspension or steering clearance.
Vehicles may use wheel-speed information for ABS, traction control, stability control and other systems.
Tire mass, sidewall height, construction and compound can change vehicle behavior even when calculated diameter is similar.
Similar Diameter Does Not Prove Fitment
This calculator compares nominal tire dimensions. It cannot determine whether a tire physically fits your vehicle.
Find Tires After Comparing Sizes
Common Tire Size Questions
How do you calculate tire diameter?
First calculate sidewall height by multiplying section width by the aspect ratio. Convert that result from millimeters to inches, multiply it by two and add the wheel diameter.
Will changing tire size affect my speedometer?
It can. Increasing overall tire diameter generally causes the vehicle to travel slightly farther per wheel revolution, while decreasing diameter does the opposite. The calculator estimates this effect using the original and new nominal diameters.
Does a larger tire increase ground clearance?
Increasing overall tire diameter can increase axle height by roughly half of the diameter increase. Actual results depend on the tire’s real measured dimensions and vehicle setup.
Does matching tire diameter guarantee a new size fits?
No. Tire width, wheel width, wheel offset, suspension clearance, steering clearance, load rating and other factors still need to be checked.
Why can actual tire dimensions differ from this calculator?
Printed tire sizes are nominal. Actual section width and overall diameter can vary between tire models and can also depend on the wheel width used for measurement.
Compare the Numbers — Then Verify the Fitment
A tire-size calculator is an excellent screening tool, but the final decision should still be checked against your vehicle requirements, wheel specifications and the exact tire manufacturer’s data.
Calculations use nominal metric tire-size dimensions and are estimates. Actual tire dimensions vary by manufacturer, tire model, inflation, measuring wheel and load. This tool does not verify vehicle fitment. Always confirm tire size, load requirements, wheel compatibility and physical clearance before purchase or installation.
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