Rolling Diameter Calculator

Calculate tire rolling diameter, rolling circumference, and revolutions per mile. Compare tire sizes and understand their effect on speedometer accuracy and gear ratios.

First number in tire size (e.g., 265/70R17)

Sidewall height as % of width

Rim size

Tire compression under vehicle weight

Measured or known circumference

Rolling Diameter Formulas: Static Diameter = Wheel Diameter + (2 * Sidewall Height) Sidewall Height = (Tire Width * Aspect Ratio) / 100 Rolling Diameter = Static Diameter * Load Factor (Load Factor: 0.95-1.00 depending on weight) Rolling Circumference = PI * Rolling Diameter Revolutions per Mile = 63,360 / Circumference (inches) Speedometer Error = ((New Diameter - Old) / Old) * 100
Example 1: Tire: 265/70R17 Sidewall: 265 * 0.70 = 185.5mm = 7.30" Static: 17 + (2 * 7.30) = 31.60" Rolling (0.97): 31.60 * 0.97 = 30.65" Circumference: PI * 30.65 = 96.3" Revs/mile: 63,360 / 96.3 = 658 Example 2 (Compare): Old: 265/70R17 = 30.65" rolling New: 285/75R17 = 32.66" rolling Difference: +2.01" (+6.56%) Speedometer: +6.56% error At 60 mph -> 63.9 mph actual WARNING: Significant - recalibration needed

What is rolling diameter and why is it different from tire diameter?

Rolling diameter is the effective diameter of a tire under load while rolling. It's smaller than static (measured) diameter because the tire compresses under vehicle weight. Example: 30" measured tire might have 29.5" rolling diameter. This affects speedometer accuracy, gear ratios, and performance calculations. Rolling circumference determines actual distance traveled per rotation.

How does tire pressure affect rolling diameter?

Lower pressure = smaller rolling diameter due to more compression. Higher pressure = larger rolling diameter, closer to static diameter. Example: 30 psi might give 29.2" rolling diameter, 40 psi gives 29.7". This affects fuel economy (underinflated = higher rolling resistance), speedometer accuracy, and handling. Check pressure when cold for accurate readings.

What is the difference between loaded and unloaded diameter?

Unloaded/Static: Measured with no weight, tire at full size. Loaded: Tire compressed under vehicle weight (typical 3-5% smaller). Rolling: Tire while moving under load (accounts for centrifugal expansion, typically 1-2% larger than loaded). Use rolling diameter for speed/distance calculations, loaded for clearance checks.

How do I use rolling diameter for gear ratio calculations?

Rolling diameter affects final drive ratio. Formula: Effective Ratio = (Engine RPM × Tire Circumference × 60) / (MPH × 336 × Gear Ratio). Larger tire = lower effective ratio = less acceleration, better fuel economy. Example: 28" to 31" tire (+10.7%) is like changing from 3.73 to 3.37 rear end.

Why is rolling circumference important?

Rolling circumference (π × rolling diameter) determines distance traveled per wheel rotation. Critical for: speedometer/odometer accuracy, gear ratio calculations, performance tuning, comparing different tire sizes. Example: 30" rolling diameter = 94.2" circumference = 672 rotations per mile. Used in ECU tuning and data logging.

How much does rolling diameter change with speed?

Centrifugal force expands tires at high speed. At 60+ mph, rolling diameter can be 0.5-1.5% larger than at low speed. Performance tires expand more. This slightly affects high-speed speedometer accuracy. Racing teams measure rolling diameter at track speed using laser sensors for precise gear ratio calculations.

What is revolutions per mile and how is it calculated?

Revs/mile shows how many times a tire rotates in one mile. Formula: 63,360 inches/mile ÷ (π × diameter in inches). Smaller tire = more revolutions. Example: 28" diameter = 720 revs/mile, 31" = 651 revs/mile. Used for speedometer calibration, gear selection, and comparing tire sizes. Critical for accurate speed readings.