Tire size comparison calculator

Compare your current tire size with a new one: overall diameter, sidewall, width, revs per mile, and speedometer difference. Free, runs in your browser, nothing is stored.

Last updated: October 12, 2026

Short answer: a size like 225/65R17 means a tire 225 mm wide, with a sidewall 65 percent as tall as that width (about 146 mm), R for radial construction, made for a 17 inch wheel. Overall diameter is the wheel plus two sidewalls, about 28.5 inches. Enter your current size and a new size below to compare them.

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How to read a tire size

  • 225: section width in millimeters, sidewall to sidewall.
  • 65: aspect ratio. Sidewall height is 65 percent of the width.
  • R: radial construction, used on nearly all modern passenger tires.
  • 17: wheel (rim) diameter in inches.
  • After the size you will usually see a service description such as 102H. That is the load index and speed rating, and any replacement should meet or exceed the original.

The math the calculator uses

  • Sidewall height = width × aspect ratio ÷ 100 (mm).
  • Overall diameter = rim inches + 2 × sidewall ÷ 25.4 (inches).
  • Circumference = π × diameter. Revs per mile = 63,360 ÷ circumference in inches.
  • True speed = speedometer reading × new diameter ÷ current diameter.

Worked example: 225/65R17 has a 146.25 mm sidewall, so its diameter is 17 + 2 × 146.25 ÷ 25.4 = 28.52 inches, a circumference of 89.6 inches and about 707 revs per mile. A 235/60R18 is 29.10 inches, 2.0 percent taller, so at an indicated 60 mph you are really doing about 61.2 mph.

Why about 3 percent matters

Planning bigger wheels? Our plus sizing guide lists the full checklist and finds sizes within 3 percent. Continental Tire’s plus sizing bulletin says the original rolling circumference should be maintained as closely as possible, because a change can affect speedometer calibration, ABS and stability control. It adds that under most circumstances a change within plus or minus 3 percent does not require adjustments to those systems, and that you should contact the vehicle manufacturer if there is any question. General Tire publishes the same guidance. The same bulletins say load capacity and speed rating must be equal to or greater than the original fitment.

A size that passes the diameter check can still be wrong for your car. Rim width, offset, brake and body clearance, and load capacity all matter. Start from your door placard, read how to choose tire size, and have a licensed installer confirm the fit.

FAQ

What does 225/65R17 mean?

225 is the section width in millimeters. 65 is the aspect ratio, so the sidewall height is 65 percent of the width, about 146 mm. R means radial construction. 17 is the wheel (rim) diameter in inches. The overall diameter works out to about 28.5 inches.

How much can tire diameter change before it is a problem?

Continental and General Tire plus sizing guidance says the original rolling circumference should be kept as closely as possible, and that under most circumstances a change within plus or minus 3 percent does not require adjustment of the speedometer, ABS or stability control. Larger changes, or any doubt, should go to the vehicle manufacturer.

Will a bigger tire make my speedometer read wrong?

Yes. A taller tire covers more ground per turn, so you travel faster than the speedometer shows. A shorter tire does the opposite. The calculator shows the true speed when the speedometer reads 30 and 60 mph.

Is this calculator exact?

No. It uses the nominal size formula. Real tires differ slightly by brand, model, tread depth and inflation, and the maker’s published revs per mile is more precise. Always confirm load index, speed rating, rim width and clearance with a licensed installer.

Do you store what I type?

No. The math runs entirely in your browser. Nothing you enter is sent to us or saved.

Sources

Outbound links to public references. We paraphrase; visit the source for full wording and updates.