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Tire Size Calculator
Your tire sizes
Example: 205/55R16 → 225/45R17: diameter 24.88 in → 24.97 in (+0.38%), and at a true 60 mph the speedometer reads 59.8 mph.
Behind the numbers
How tire size is calculated
A tire size like 205/55R16 packs three numbers into one code. The sidewall height is the width times the aspect ratio, and the overall diameter is the rim diameter plus two sidewalls. Change any of the three numbers and the diameter, circumference and speedometer reading all change with it.
205/55R16: 16 + 2 × (205 × 55 ÷ 100 ÷ 25.4) = 16 + 2 × 4.44 = 24.88 in. 225/45R17: 17 + 2 × (225 × 45 ÷ 100 ÷ 25.4) = 17 + 2 × 3.99 = 24.97 in.
How to read a tire size
The width is the tread width in millimeters, the aspect ratio is the sidewall height as a percentage of the width, R means radial construction, and the last number is the wheel (rim) diameter in inches. The full size is printed on the tire sidewall and on the sticker inside the driver's door.
A few comparisons
Example tire swaps
| Swap | Old diameter | New diameter | Change | Verdict |
|---|---|---|---|---|
| 195/65R15 → 205/55R16 | 24.98 in | 24.88 in | -0.41% | OK |
| 205/55R16 → 225/45R17 | 24.88 in | 24.97 in | +0.38% | OK |
| 225/65R17 → 265/70R17 | 28.52 in | 31.61 in | +10.84% | Over 3% - check fit |
| 235/45R18 → 245/40R18 | 26.33 in | 25.72 in | -2.32% | OK |
Small change. Big difference.
Keep the diameter change within ±3% of the original size. Bigger changes throw off the speedometer and odometer, and can rub against the fender or suspension.
A wider tire is not automatically taller. A 225/45R17 is almost the same diameter as a 205/55R16 because the lower aspect ratio cancels out the extra width.
Check the sticker inside the driver's door or your owner's manual for the factory size before you shop — that is the size your speedometer was calibrated for.
When the diameter grows, the speedometer reads low: at a true 60 mph with the example swap it reads 59.8 mph, so you are going slightly faster than the needle shows.
Good to know
Your questions, answered.
What is the 3% rule for tire sizes?
Most tire makers and mechanics recommend keeping the new tire's overall diameter within 3% of the original. A 205/55R16 to 225/45R17 swap is only a 0.38% change, so it stays well inside that range.
Does a bigger tire change my speedometer reading?
Yes. The speedometer is calibrated to the original tire's revolutions per mile. A taller tire turns fewer times per mile, so the speedometer reads low: with the example swap, at a true 60 mph the speedometer reads 59.8 mph.
Can I put a different tire size on the same rim?
Only sizes made for that rim diameter fit - the last number (16 in 205/55R16) must match your wheel. Width and aspect ratio can change within limits, but the tire must still clear the fenders and suspension.
What does 205/55R16 mean?
205 is the tread width in millimeters, 55 is the aspect ratio (the sidewall height is 55% of the width), R means radial construction, and 16 is the rim diameter in inches. The example tire has a 4.44-inch sidewall and a 24.88-inch overall diameter.