Tire Circumference Calculator

Related Calculators

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Tire size code: The sidewall code that gives the tire dimensions, such as 205/55R16 or 31x10.50R15.

Sidewall height: The rubber height from the wheel rim to the tread. In a metric code, it comes from tire width times aspect percent.

Overall tire diameter: The outside height of the tire from tread to tread through the center.

Tire circumference: The distance around the outside of the tire. A larger circumference rolls farther each turn.

Tire turns per mile: The estimated number of full tire rotations needed to travel one mile.

Circumference change: The percent difference between the new tire circumference and the original tire circumference.

Ride-height change: Half of the diameter change. Positive raises the axle, and negative lowers it.

Dashboard speed error: Dashboard speed minus estimated real speed. Positive means the dashboard reads faster than the road speed.

Mistake: Typing only the rim size in Tire size code , such as 16.
Fix: Enter the full sidewall code in Tire size code , such as 205/55R16 or 31x10.50R15.

Mistake: Using the new tire in Original tire size code when checking a swap.
Fix: Put the factory or current calibrated size in Original tire size code and the replacement size in New tire size code .

Mistake: Reading Circumference change as a direct ride-height change.
Fix: Use Ride-height change ; it is based on half of the diameter difference, not the full circumference difference.

Mistake: Leaving Dashboard speed to check at 60 mph when you care about a different speed.
Fix: Change Dashboard speed to check to the speed you want, then read Real speed at your dashboard speed and Dashboard speed error .

Mistake: Entering a zero or negative number in Dashboard trip distance to check .
Fix: Use a positive trip distance in Dashboard trip distance to check , such as 100, so Real distance for your odometer trip can be calculated.

• The calculator uses nominal tire dimensions from the size code. Real rolling circumference can change with tire model, load, inflation pressure, tread wear, and measuring rim width.
• It does not check rim width range, load index, speed rating, brake clearance, suspension clearance, ABS rules, traction-control rules, or manufacturer approval.
• Metric tire codes are calculated from width, aspect percent, and rim diameter. Flotation codes such as 31x10.50R15 already state outside diameter, so the calculator uses that stated diameter.
• Compare mode assumes the speedometer and odometer were calibrated for the Original tire size code.
• Road speed and odometer results are estimates. Vehicle electronics, tire growth at speed, and tire wear can make the real value different.
• A small percent change can still matter for clearance, gearing feel, warranty rules, or tire-chain fit, so check your vehicle manual or tire professional before buying.

How the size code is read

For a metric code such as 205/55R16, the calculator reads 205 as width in millimeters, 55 as aspect percent, and 16 as rim diameter in inches. Common prefixes such as P, LT, ST, and T do not change the dimension math. Load and speed text after the rim size is ignored when the size part is valid.

For a flotation code such as 31x10.50R15, the calculator reads 31 as outside diameter in inches, 10.50 as section width in inches, and 15 as rim diameter in inches.

Core formulas

sidewall_mm = width_mm * (aspect_percent / 100)

diameter_mm = (rim_in * 25.4) + (2 * sidewall_mm)

diameter_mm = flotation_diameter_in * 25.4

circumference_mm = pi * diameter_mm

circumference_in = circumference_mm / 25.4

revs_per_mile = 1609344 / circumference_mm

revs_per_km = 1000000 / circumference_mm

Compare formulas

circ_change_percent = ((new_circumference_mm - original_circumference_mm) / original_circumference_mm) * 100

actual_speed_mph = dashboard_speed_mph * (new_circumference_mm / original_circumference_mm)

speedometer_error_mph = dashboard_speed_mph - actual_speed_mph

actual_distance_miles = dashboard_trip_miles * (new_circumference_mm / original_circumference_mm)

ride_height_change_mm = (new_diameter_mm - original_diameter_mm) / 2

Mini-example

For 205/55R16, sidewall height is 205 * 0.55 = 112.75 mm. The 16 inch rim is 406.4 mm, so diameter is 406.4 + 2 * 112.75 = 631.9 mm. Circumference is pi * 631.9 = about 1985.17 mm, or about 78.16 inches. That gives about 810.68 tire turns per mile.

For a swap from 205/55R16 to 215/55R16, the new circumference is about 1.74% larger. At a 60 mph dashboard reading, the estimated real speed is 60 * 1.0174 = about 61.04 mph, and the dashboard speed error is about -1.04 mph.