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Tire Size Calculator

Compare your current tires with a new size to see how the diameter, sidewall and circumference change, whether the swap stays within 3%, and how fast you're really going when the speedometer reads 60.

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How to use the tire size calculator

  1. Enter your current tire size exactly as it appears on the sidewall, such as 225/45R17. P and LT prefixes and load or speed codes after a space are fine.
  2. Enter the new tire size you're considering.
  3. Enter a speedometer reading in mph or km/h to see your true speed on the new tires.
  4. Read the diameter difference, the side-by-side comparison and the speedometer table.

How tire dimensions are calculated

A size like 225/45R17 packs in three numbers. 225 is the section width in millimeters. 45 is the aspect ratio: the sidewall height as a percentage of the width. R means radial construction, and 17 is the wheel diameter in inches.

Sidewall height = width × aspect ratio ÷ 100 (mm) Overall diameter = wheel diameter × 25.4 + 2 × sidewall (mm) Circumference = π × overall diameter Revolutions per mile = 1,609,344 mm ÷ circumference Actual speed = speedometer reading × new diameter ÷ old diameter

The sidewall counts twice because there's one above the wheel and one below it. Your speedometer counts wheel turns, so a taller tire covers more ground per turn than the speedometer expects and you travel faster than it shows.

Worked example

The defaults compare 225/45R17 with 235/40R18:

  • Current sidewall: 225 × 45 ÷ 100 = 101.25 mm
  • Current diameter: 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm (24.97 in)
  • New sidewall: 235 × 40 ÷ 100 = 94 mm
  • New diameter: 18 × 25.4 + 2 × 94 = 457.2 + 188 = 645.2 mm (25.40 in)
  • Difference: 645.2 ÷ 634.3 − 1 = +1.72%, inside the ±3% guideline
  • At an indicated 60 mph you're doing 60 × 645.2 ÷ 634.3 = 61.0 mph
  • Revolutions per mile fall from 807.6 to 794.0

Going down to 205/55R16 gives a diameter of 631.9 mm, a difference of −0.38%, so the speedometer would read very slightly fast instead.

The ±3% guideline

Most tire shops and many carmakers suggest keeping a replacement tire's overall diameter within about 3% of the original. A bigger change can affect:

  • Speedometer and odometer accuracy. A 3% taller tire reads 58.2 mph when you're doing 60.
  • Clearance. A wider or taller tire can rub the fender liner, suspension or brake parts when you turn or hit a bump.
  • Stability and ABS systems, which compare wheel speeds and expect the original size.
  • Gearing and fuel economy. Taller tires make the engine work harder from a stop.

The guideline is a rule of thumb, not a law. Your owner's manual and the tire placard on the driver's door jamb list the sizes approved for your vehicle, and a tire professional can confirm whether another size fits. Load index and speed rating must also be at least as high as the original's.

Plus sizing

"Plus one" or "plus two" means going up one or two inches in wheel diameter while lowering the aspect ratio so the overall diameter stays close. The default comparison, from 17 to 18 inches with the aspect ratio dropping from 45 to 40, is a plus-one swap. The lower sidewall usually sharpens steering response, at the cost of a firmer ride and wheels that are easier to damage on potholes.

Frequently asked questions

What do the numbers on a tire mean?

In 225/45R17 94W, 225 is the width in mm, 45 is the sidewall height as a percentage of the width, R means radial, and 17 is the wheel size in inches. 94 is the load index and W is the speed rating. Only the first three numbers change the tire's dimensions.

Will bigger tires make my speedometer wrong?

Yes, if the overall diameter grows. The speedometer will read low: with tires 1.72% taller, it shows 60 mph when you're doing 61.0 mph. Many shops can recalibrate the speedometer after a big size change.

Can I put wider tires on the same wheels?

Often by one size step, such as 225 to 235, if the wheel width is within the tire maker's approved range. Check the manufacturer's rim width chart and make sure the tire clears the suspension and fenders at full steering lock.

How many revolutions per mile does my tire make?

Divide 1,609,344 (the millimeters in a mile) by the circumference in millimeters. A 225/45R17 tire has a circumference of 1,992.7 mm, so it turns about 807.6 times per mile. Real figures are a little higher, because the tire flattens slightly under the car's weight.

Related calculators

  • Gas Mileage Calculator — Your car's real fuel economy in mpg (US and UK), L/100 km and km/L from two fill-ups, plus cost per mile.
  • Speed Calculator — Speed from distance and time, distance from speed and time, or time from distance and speed, in any unit.
  • Unit Conversion Calculator — Convert length, weight, temperature, area, volume, speed, time, data, pressure, energy, power and fuel economy.

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