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All calculatorsMethodology

Tire Size Calculator

Compare two tire sizes by width, aspect ratio and rim diameter: sidewall, overall diameter, circumference, revolutions per mile and speedometer error.

Interactive calculator · Method TIRE-SIZE-1.0 · Last reviewed: 2026-11-04

Calculate your scenario

Sidewall height = width × aspect ratio ÷ 100 (mm).

Your inputs

Try an example

Default worked example. Edit the inputs to calculate your scenario.

Default worked example

Actual speed with new tire 60.23 mph
Overall diameter difference +0.38%
Overall diameter (original → new) 631.9 mm (24.88 in) → 634.3 mm (24.97 in)
Sidewall height (original → new) 112.8 mm (4.44 in) → 101.3 mm (3.99 in)
Revolutions per mile (original → new) 810.7 → 807.6

Diameter = rim × 25.4 + 2 × width × aspect ÷ 100: 631.9 mm → 634.3 mm, so a reading of 60 mph is 60.23 mph on the new tire.

How to use the calculator

Enter the original size from the sidewall (for 205/55R16: width 205, aspect 55, rim 16), the new size, and a speedometer reading to see the true speed with the new tire.

What the result means

Going from 205/55R16 to 225/45R17 makes the tire about 2.4 mm (0.38%) taller, so 60 mph on the speedometer is about 60.23 mph on the road.

Formula and units

Sidewall height = width × aspect ratio ÷ 100 (mm). Overall diameter = rim diameter × 25.4 + 2 × sidewall height. Circumference = π × diameter; revolutions per mile = 1,609,344 mm ÷ circumference. Actual speed = speedometer reading × new diameter ÷ original diameter, assuming the speedometer reads correctly on the original tire.

Worked examples

205/55R16 to 225/45R17 at 60 mph: Original tire width: 205 mm; Original aspect ratio: 55 %; Original rim diameter: 16 in; New tire width: 225 mm; New aspect ratio: 45 %; New rim diameter: 17 in; Speedometer reading: 60 mph or km/h; Speed unit: mph.
Example result: Actual speed with new tire: 60.23 mph; Overall diameter difference: +0.38%; Overall diameter (original → new): 631.9 mm (24.88 in) → 634.3 mm (24.97 in); Sidewall height (original → new): 112.8 mm (4.44 in) → 101.3 mm (3.99 in); Revolutions per mile (original → new): 810.7 → 807.6.

265/70R17 to 285/75R17 at 100 km/h: Original tire width: 265 mm; Original aspect ratio: 70 %; Original rim diameter: 17 in; New tire width: 285 mm; New aspect ratio: 75 %; New rim diameter: 17 in; Speedometer reading: 100 mph or km/h; Speed unit: km/h.
Example result: Actual speed with new tire: 107.04 km/h; Overall diameter difference: +7.04%; Overall diameter (original → new): 802.8 mm (31.61 in) → 859.3 mm (33.83 in); Sidewall height (original → new): 185.5 mm (7.3 in) → 213.8 mm (8.42 in); Revolutions per mile (original → new): 638.1 → 596.1.

Limitations and common mistakes

Nominal, unloaded dimensions from the size code. Real tires differ by brand, tread depth, inflation and load, so measured rolling circumference can be a few percent smaller. This does not judge fitment, clearance, load index, speed rating, braking or inflation pressure; follow the vehicle and tire maker’s guidance.

Treating the aspect ratio as a height in millimeters. 55 means the sidewall is 55% of the width, so a wider tire with the same aspect ratio also gets taller.

Sources and assumptions

Method TIRE-SIZE-1.0; last formula review 2026-11-04. Sources and their supported claims. Calculation methodology.