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Tire Size Calculator | Diameter, Revs & Speedo

A tire size calculator decodes metric tire codes (width/aspect/R rim) into overall diameter: rim inches × 25.4 + 2 × sidewall, where sidewall = width × aspect ÷ 100. It outputs diameter, sidewall height, revolutions per mile and the speedometer deviation versus a stock size.

Last updated: September 2026 · Free · No sign-up required

Results

Enter values above and click Calculate to see your result instantly.

Quick reference

SizeDiameterRevs/mile
205/55R16632 mm / 24.9 in810
225/45R17634 mm / 25.0 in808
245/40R18653 mm / 25.7 in788
265/70R17803 mm / 31.6 in640

straighten The 3% rule

Keep total diameter within ±3% of stock so gearing, ABS and speed calibration stay within tolerance.

speed Speedo error math

Speedometer error equals the diameter change: a 5% taller tire makes the speedo under-read by 5%.

sync Revs per mile

Fewer revs per mile = taller tire = gentler cruising revs but slower acceleration and higher effective gearing.

warning Fitment check

Diameter is only half the story — check wheel width, offset and body clearance for rubbing before buying.

lightbulb A real example — plus-sizing a hot hatch

Stock 205/55R16 (632 mm) replaced with 225/40R17.

Result: New diameter = 431.8 + 2 × 90 = 612 mm — 3.2% smaller; the speedo now over-reads by 3.2%.

What this means: That's right at the tolerance edge. One step wider with a slightly taller aspect (225/45R17, 634 mm) keeps almost perfect speed calibration.

Formula & methodology

Formula: Diameter(mm) = Rim(in) × 25.4 + 2 × Width × Aspect ÷ 100; Revs/mile = 63,360 ÷ (Diameter in inches × π)

Reading the code 225/45R17

  • 225 — section width in mm.
  • 45 — aspect ratio: sidewall is 45% of width = 101 mm.
  • 17 — wheel diameter in inches = 431.8 mm.

Overall diameter = 431.8 + 2 × 101 ≈ 634 mm (25.0 in).

Revolutions per mile = 63,360 ÷ (25.0 × π) ≈ 808. A taller replacement tire makes the speedometer read low by the same percentage the diameter grew: +3% diameter → speedo shows 60 when you're doing 61.8.

Frequently asked questions

How do you calculate tire diameter from the code?
Multiply the rim by 25.4 to get mm, add twice the sidewall (width × aspect ÷ 100). For 225/45R17: 431.8 + 2 × 101.25 ≈ 634 mm.
What size is 225/45R17 in inches?
About 25.0 inches overall diameter, with an 8.9 in section width and a 4.0 in sidewall.
Do bigger tires affect the speedometer?
Yes — the speedo under-reads by the diameter increase. A 3% taller tire shows 60 mph when true speed is 61.8 mph.
How much tire size change is safe?
Stay within ±3% of the stock diameter. Beyond that, expect gearing, odometer, ABS/ESP and clearance issues.

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