TireMath

Tire Size Calculator

Decode a tire size (e.g. 225/45R17) into overall diameter, and see how a size change affects your speedometer.

Original tire (width / aspect / rim)

New tire (width / aspect / rim)

Indicated speed on speedometer (mph)

Original diameter: 24.97 in
New diameter: 25.4 in
Diameter change: 1.72%
Actual speed at new size: 66.1 mph

What you entered

Original tire: 225/45R17New tire: 235/40R18Indicated speed: 65 mph

1
Original tire diameter
17 + 2 × (225 × 45% ÷ 25.4)= 24.97 in
2
New tire diameter
18 + 2 × (235 × 40% ÷ 25.4)= 25.40 in
3
Diameter change
(25.40 − 24.97) ÷ 24.97= 1.72%
4
Actual speed at new tire size
65 × (25.40 ÷ 24.97)= 66.1 mph

Result

Actual speed: 66.1 mph

Swapping to a 235/40R18 tire changes the rolling diameter by 1.72% — at an indicated 65 mph, you're actually going 66.1 mph.

Diameter change and speedometer effect for common tire upsizes

Each row reads two tire sizes the way the sidewall is written: width in mm, sidewall height as a percentage of that width, then rim diameter in inches. Overall diameter is the rim plus two sidewalls, and the change column is what your speedometer error becomes after the swap.

Original tire	New tire	Original diameter (in)	New diameter (in)	Diameter change
195/65R15	205/55R16	24.98	24.88	-0.41%
205/55R16	225/45R17	24.88	24.97	0.38%
215/60R16	225/50R17	26.16	25.86	-1.14%
245/45R18	255/40R19	26.68	27.03	1.31%
225/45R17	235/40R18	24.97	25.40	1.72%
225/65R17	235/60R18	28.52	29.10	2.06%
235/75R17	265/70R17	30.88	31.61	2.36%
265/70R17	285/70R17	31.61	32.71	3.49%
275/55R20	285/60R20	31.91	33.46	4.87%

A speedometer is calibrated to the original rolling diameter, so a larger tire means you are travelling faster than the dial shows by the same percentage: at an indicated 65 mph the 3.49 percent row is really doing about 67.3 mph, and the 4.87 percent row about 68.2 mph. Most manufacturers advise staying within 3 percent of the original diameter, which the last two rows exceed, so those swaps can also upset ABS and stability control calibration, odometer and trip readings, and transmission shift points. Diameter is only half the question: check load index, speed rating, wheel width and offset, and confirm the tire clears the fender and suspension at full lock and full compression before buying.

Reading a tire size

225/45R17 means: 225mm wide, sidewall height is 45% of that width, mounted on a 17-inch rim. Change any of these and the overall rolling diameter changes, throwing off your speedometer slightly.

Frequently asked questions

I'm switching from 225/45R17 to 235/40R18 — will my speedometer be off?

Enter both sizes above to see the exact difference. In this case, the overall diameters are very close (24.97" vs. 25.04"), so speedometer error would be under 0.3% — effectively negligible.

How do I read the numbers on the side of my tire?

Take 225/45R17: 225 is the width in mm, 45 is the aspect ratio (sidewall height is 45% of the width = 101mm), R means radial construction, and 17 is the rim diameter in inches. Overall diameter = rim + 2 sidewalls.

How much tire size change is too much?

Most manufacturers recommend staying within 3% of the original overall diameter. Larger changes can affect speedometer accuracy, ABS calibration, and transmission shift points. Some changes may also rub against fenders or suspension components.