BYTETOOLS

Tire Size Calculator

Decode any metric or flotation tyre size into sidewall height, overall diameter and revolutions per mile, and compare two sizes for speedometer error.

24.97
Diameter (in)
3.99
Sidewall (in)
78.5
Circumference (in)
808
Revs per mile

225/45R17 in detail

  • Section width225 mm (8.86 in)
  • Aspect ratio45.0%
  • Rim diameter17 in
  • Sidewall height3.986 in (101.3 mm)
  • Overall diameter24.972 in (634.3 mm)
  • Circumference78.45 in (1,992.7 mm)
  • Revolutions per mile807.6
  • Revolutions per km501.8

Sidewall = section width × aspect ÷ 100. Overall diameter = rim + 2 × sidewall. Revolutions per mile = 63,360 ÷ circumference — the free-rolling geometric figure. Manufacturers publish a slightly higher number because a loaded tyre squashes and rolls a shorter distance per turn.

225/45R17 versus 245/40R18

  • Diameter change: +2.98%
  • Ride height: +0.372 in at the axle
  • Section width: +20 mm
  • Revs per mile: 784.2 (was 807.6)
  • Odometer reads 97.1% of the true distance

Speedometer error

With the new tyres, an indicated 60 mph is really 61.8 mph — out by 1.8 mph on the fast side.

Fitters generally keep the diameter change inside ±3% so the speedometer, ABS and traction control stay within their calibration. This pair is +2.98% — inside that window.

What is the Tire Size Calculator?

The ByteTools Tire Size Calculator reads both size formats. A metric size like 225/45R17 gives a 225 mm section width, a sidewall 45% of that, and a 17 inch rim; a flotation size like 33x12.

  • Parses metric (P225/45R17, LT265/70R17) and flotation (33x12.50R15) sizes
  • Sidewall height, overall diameter and circumference in both inches and millimetres
  • Revolutions per mile and per kilometre from the exact 63,360 inches-per-mile constant
  • Side-by-side comparison with percentage diameter change and ride-height delta
  • Speedometer and odometer error at any indicated speed
  • Both tyres drawn to scale on top of each other, with the ±3% fitment window flagged

How to use the Tire Size Calculator

  1. 1

    Type your current tyre size, such as 225/45R17 or 33x12.50R15 — a load and speed rating on the end is fine.

  2. 2

    Type the size you are considering into the second field.

  3. 3

    Enter the speed shown on your speedometer and pick mph or km/h.

  4. 4

    Read the diameter, sidewall, circumference and revolutions per mile in the tiles.

  5. 5

    Check the comparison panel for the diameter change, ride-height delta and real speed.

About the Tire Size Calculator

The ByteTools Tire Size Calculator reads both size formats. A metric size like 225/45R17 gives a 225 mm section width, a sidewall 45% of that, and a 17 inch rim; a flotation size like 33x12.50R15 states the overall diameter in inches directly. From either one the tool works out sidewall height, overall diameter, circumference, and revolutions per mile as 63,360 ÷ circumference.

Enter a second size and it becomes a comparison. You get the percentage change in diameter, the change in ride height at the axle, the difference in section width, and the speedometer error — with the new tyres fitted, what an indicated speed really means. Fitters generally keep the diameter change inside ±3%, and the tool tells you which side of that line you are on.

Everything is computed in your browser from the size codes alone, so nothing is uploaded and the page works offline. One honest caveat: revolutions per mile here is the free-rolling geometric figure, 63,360 divided by the circumference. A loaded tyre squashes slightly and turns a few more times per mile, which is why manufacturers publish a number two to four per cent higher.

Frequently asked questions

How do you calculate tire diameter from the size?

Sidewall height is the section width times the aspect ratio divided by 100, converted to inches. Overall diameter is the rim diameter plus twice that sidewall. For 225/45R17: 225 × 0.45 = 101.25 mm = 3.99 in of sidewall, so the diameter is 17 + 7.97 = 24.97 inches.

What does 225/45R17 mean?

225 is the section width in millimetres, 45 is the aspect ratio — the sidewall is 45% of the width — R means radial construction, and 17 is the wheel diameter in inches. A letter in front, like P for passenger or LT for light truck, indicates the intended service.

How much does tire size affect the speedometer?

Speedometer reading scales directly with tyre diameter. Fitting a tyre 3% taller means your true speed is 3% higher than indicated, so 60 mph shown is really 61.8 mph. The calculator works this out for whatever indicated speed you enter.

How much bigger can I go on tire size?

Most fitters keep the overall diameter within about 3% of the original so the speedometer, ABS and traction control stay inside their calibration, and so the tyre clears the arch on full lock. The tool shows the percentage change and says whether it falls inside that window.

Why is the revolutions per mile figure lower than the manufacturer's?

Because this is the geometric figure, 63,360 divided by the free circumference. A loaded tyre deflects, so its rolling radius is a little smaller and it turns a few more times per mile — typically 2 to 4% more than the geometric value. Manufacturers publish the measured loaded figure.

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