Skip to content
CalcBro

Tire Size Calculator

Compare two tire (tyre) sizes side by side: overall diameter, sidewall height, circumference, revolutions per km and mile, and the effect on your speedometer.

Current tire
millimetres
percent
inches
New tire
millimetres
percent
inches

Results

Diameter difference

2.98% larger

The new tire is 2.98% larger in diameter than the current tire. Your actual speed would be 2.98% higher than the speedometer shows.

Current diameter
634.3 mm
New diameter
653.2 mm
Speedometer shows 100 km/h
102.98 km/h actual
Speedometer shows 60 mph
61.79 mph actual
Tire dimensions
Current225/45 R17New245/40 R18
Sidewall (mm)101.2598
Diameter (mm)634.3653.2
Diameter (in)24.9725.72
Circumference (mm)1,992.72,052.1
Revs per km501.8487.3
Revs per mile807.6784.2

The new tire is 2.98% larger in diameter than the current tire. Current diameter 634.3 millimetres, new diameter 653.2 millimetres. Your actual speed would be 2.98% higher than the speedometer shows.

Formula

Sidewall height (mm)
= width (mm) × aspect ratio (%) ÷ 100
Rim diameter (mm)
= rim diameter (in) × 25.4
Overall diameter (mm)
= rim diameter (mm) + 2 × sidewall height (mm)
Circumference (mm)
= π × overall diameter (mm)
Revolutions per km
= 1,000,000 ÷ circumference (mm)
Revolutions per mile
= 1,609,344 ÷ circumference (mm)
Diameter difference (%)
= (new diameter − current diameter) ÷ current diameter × 100
Actual speed
= speedometer reading × new circumference ÷ current circumference

The sidewall appears twice in the overall diameter because there is one above the rim and one below it. Circumference is π times the diameter, so the circumference and speed change by exactly the same percentage as the diameter.

Worked example

You are replacing 225/45 R17 tires with 245/40 R18.

  1. Step 1: 225/45 R17 sidewall

    225 × 45 ÷ 100 = 101.25 mm

  2. Step 2: 225/45 R17 diameter

    17 × 25.4 + 2 × 101.25 = 634.3 mm (24.97 in)

  3. Step 3: 245/40 R18 sidewall

    245 × 40 ÷ 100 = 98 mm

  4. Step 4: 245/40 R18 diameter

    18 × 25.4 + 2 × 98 = 653.2 mm (25.72 in)

  5. Step 5: Diameter difference

    (653.2 − 634.3) ÷ 634.3 × 100 = 2.98% larger

  6. Step 6: Actual speed at an indicated 100 km/h

    100 × 653.2 ÷ 634.3 = 102.98 km/h

The new tire is 2.98% larger in diameter than the current tire. When the speedometer shows 100 km/h you would be doing about 102.98 km/h, and at 60 mph about 61.79 mph, assuming the speedometer was accurate on the original tires.

How to read a tire size

A size such as 225/45 R17 is printed on the sidewall of every tire (or tyre, in British and Australian spelling). It describes the tire's nominal dimensions:

  • 225 is the section width in millimetres.
  • 45 is the aspect ratio: the sidewall height as a percentage of the width.
  • R means radial construction. It is not the radius.
  • 17 is the diameter of the wheel (rim) in inches.
  • Any code after the size, such as 91W, is the load index and speed rating. They do not affect the dimensions, so the calculator does not ask for them.

What the speedometer difference means

A speedometer works out speed from how fast the wheels turn. A larger tire travels further with every turn, so at the same wheel speed the car is moving faster than the speedometer shows. A smaller tire has the opposite effect. The odometer is affected by the same percentage.

The calculator assumes your speedometer was accurate with the current tires. Real speedometers have their own calibration and tolerances, which it does not model.

Why the figures are approximate

Tire sizes are nominal. Two tires of the same size from different manufacturers can differ slightly in real dimensions, a loaded tire's rolling radius is smaller than its unloaded radius, and tread wear reduces the diameter over time. Treat the results as a comparison of the nominal sizes rather than an exact measurement.

What this calculator does not check

It compares tire geometry only. It cannot tell you whether a size fits your vehicle or is suitable for it. In particular, it does not check:

  • clearance to the suspension, brakes, bodywork and wheel arches, including when steering or under compression;
  • whether the vehicle manufacturer approves the size;
  • whether the load index and speed rating are appropriate;
  • whether the combination is legal for your vehicle where you live.

Frequently asked questions

How do I calculate tire diameter?

Multiply the width by the aspect ratio and divide by 100 to get the sidewall height, then add two sidewalls to the rim diameter in millimetres (inches × 25.4). For 225/45 R17: 225 × 45 ÷ 100 = 101.25 mm, and 17 × 25.4 + 2 × 101.25 = 634.3 mm, or 24.97 inches.

How do larger tires affect my speedometer?

If the new tires have a larger diameter, you travel faster than the speedometer shows, by the same percentage as the diameter difference. Going from 225/45 R17 to 245/40 R18 is 2.98% larger, so an indicated 100 km/h is about 102.98 km/h.

Is this the same as a tyre size calculator?

Yes. Tire and tyre are the American and British spellings of the same word, and the sizes are written the same way everywhere, so the calculator works for any metric tire size.

How do I calculate revolutions per mile?

Divide 1,609,344 (the millimetres in a mile) by the tire's circumference in millimetres. A 225/45 R17 tire has a circumference of about 1,992.7 mm, so it turns about 807.6 times per mile, or 501.8 times per kilometre.

Can I use tire sizes like 31x10.50R15?

Not directly. The calculator uses metric sizes (width in mm, aspect ratio and rim in inches). A flotation size like 31x10.50R15 already gives the overall diameter: about 31 inches.

Will the new tire size fit my car?

The calculator cannot tell you. It compares dimensions only. Check your vehicle's approved sizes in the owner's manual or door-jamb placard, and ask a tire professional about clearance, load index and speed rating.

Related calculators

Browse all automotive calculators