Tire Size Calculator
Compare Two Tires
Comparison Result
| Dimension | Tire 1 | Tire 2 | Difference |
|---|---|---|---|
| Diameter | — | — | — |
| Width | — | — | — |
| Sidewall | — | — | — |
| Circumference | — | — | — |
| Revs/Mile | — | — | — |
How to Use This Tire Size Calculator
Tire sidewalls mix millimeters, percentages, and inches on one label—and a "one size up" wheel upgrade can rub fenders or throw off your speedometer before you notice on the highway. I run this comparison before buying anything that is not an exact OEM replacement.
- Enter Tire 1 (Original). Input width (mm), aspect ratio (%), and wheel diameter (inches) from the placard or sidewall—e.g., 225/50R17.
- Enter Tire 2 (New). Type the upgrade or alternate size you are considering.
- Press Compare. Review diameter, width, sidewall height, circumference, revolutions per mile, and speedometer error.
Larger rolling diameter can slightly improve highway economy but may affect gearing—cross-check real-world changes with the Gas Mileage Calculator after installation.
Tire Size Formulas and Practical Applications
A tire's aspect ratio is sidewall height as a percent of section width—like sole thickness relative to shoe width. Total diameter sets how far the car moves per revolution, which drives speedometer accuracy and fender clearance.
Sidewall height
Sidewall (in) = (Width × Aspect Ratio ÷ 100) ÷ 25.4
Overall diameter
Overall Diameter = (Sidewall × 2) + Wheel Diameter
For 225/50R17: sidewall = (225 × 0.50) ÷ 25.4 ≈ 4.43"; diameter ≈ 4.43 × 2 + 17 ≈ 25.9". Circumference follows π × diameter, and revolutions per mile equals 63,360 ÷ circumference (inches in a mile).
Speedometer correction
True speed scales with diameter ratio. If Tire 2 is 2% taller, a displayed 60 mph reads as roughly 61.2 mph actual—the warning box shows this at a 60 mph reference point.
Standard Units and Conversion Tables
Passenger tire labels follow ISO metric sizing. Each field in the calculator maps to one segment of the sidewall code.
- Width (mm): Section width at the widest point—first number in 225/50R17.
- Aspect ratio (%): Sidewall height as percent of width—the middle number.
- Wheel diameter (in): Rim size the tire mounts on—the number after R.
- Output dimensions: Diameter, width, sidewall, and circumference in inches; revs/mile for odometer and ABS calibration context.
Safe diameter change rule of thumb
Aim to stay within about ±3% overall diameter vs stock. Beyond that, ABS pulse rates, traction control, and automatic shift points may drift from factory calibration.
Frequently Asked Questions
How much can I change my tire size safely?
Stay within about 3% total diameter change from factory size when possible. Larger deviations can affect ABS, traction control, gearing, and clearance.
Why does a larger tire change my speedometer reading?
Speed is inferred from wheel revolutions. A taller tire travels farther per revolution, so the speedometer reads low—you may be going faster than the dash shows.
What do the three numbers on a tire mean?
A label like 225/50R17 means 225 mm section width, 50% aspect ratio sidewall height, and 17-inch wheel diameter. Enter those three values for each tire you compare.
What is plus sizing?
Plus sizing keeps overall diameter close to stock while fitting a larger wheel and lower-profile tire—wider contact patch and shorter sidewall with similar rolling circumference.
How is speedometer error calculated here?
Actual speed scales with the ratio of new tire diameter to original diameter. If Tire 2 is 2% taller, a speedometer reading of 60 mph corresponds to about 61.2 mph true speed.