Why is the middle number a percentage?

Because it is a ratio, not a length. A 45 on a 225 tire is a sidewall of 101 mm; the same 45 on a 275 tire is 124 mm. That is why swapping to a wider tire without lowering the aspect ratio makes the whole wheel taller, and why plus-sizing pairs a wider tire with a smaller second number.

How far can the diameter change before it matters?

About 3% is the figure the trade works to, and it is a practical limit rather than a legal one. Past it the speedometer error becomes noticeable, the odometer undercounts or overcounts mileage, and systems that infer wheel slip from rotation speed are working from a wrong assumption.

Does a bigger tire make the speedometer read high or low?

Low. A taller tire covers more ground per turn, so the car is going faster than the dial says — the reading is behind reality. A smaller tire does the opposite and flatters you. The figure above shows the true speed for whatever the dial reads.

Why are the tire in millimetres and the rim in inches?

Because the code is a hybrid that nobody redesigned. Section width and sidewall are metric; rim diameter stayed in inches because rims were standardised first and the wheels did not change when tire labelling did. Every metric tire size carries both systems in one string, and the bridge between them is the inch, which is a defined length in millimetres rather than a measured one.

Tire Size Calculator

With current width 225 mm, current aspect ratio 45 %, current rim 17 in, new width 245 mm, tire size comes to 2.98 % — difference in diameter. It is reached in 9 steps, the last of which is (diameter_b - diameter_a) / diameter_a * 100, and each one is printed on the page with its numbers filled in. The formula is the one published by 49 CFR 571.109, not an approximation fitted to it.

What a tire size means in inches, and what changing it does to the rolling diameter and to the speedometer reading.

By Alex Seote, Built and maintains Rule Calculator · Formula and sources checked 16 August 2026 · How we check

Current width 225 mm, Current aspect ratio 45 %, Current rim 17 in, New width 245 mm

2.98 %

Difference in diameter for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.

It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.

Current widthmmThe first number on the sidewall: 225/45R17 is 225 mm across the section.Current aspect ratio%The second number. It is a percentage of the width, not a length — 45 means the sidewall is 45% of 225 mm.Current riminThe number after the R. Rims are measured in inches even on a metric tire.New widthmmNew aspect ratio%New riminSpeedometer readsmph

Difference in diameter

2.98 %

Current sidewall height: width_a * aspect_a / 100 / 25.43.986 in
Current overall diameter: rim_a + 2 * sidewall_a24.972 in
New sidewall height: width_b * aspect_b / 100 / 25.43.858 in
New overall diameter: rim_b + 2 * sidewall_b25.717 in
Difference in diameter: (diameter_b - diameter_a) / diameter_a * 1002.98 %
New rolling circumference: pi * diameter_b80.791 in
Current revolutions per mile: 63360 / (pi * diameter_a)807.615
New revolutions per mile: 63360 / circumference_b784.247
True speed when the dial reads that: speed * diameter_b / diameter_a61.788 mph

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Worked example

225/45R17 stands 24.97 in tall: a sidewall of 225 x 0.45 = 101.25 mm, which is 3.99 in, twice over plus a 17 in rim. Going to 245/40R18 gives 25.72 in — 3.0% larger, right at the limit the trade works to, and the speedometer then reads 60 when the car is doing 61.8.

How to work it out yourself

1.Read the sidewall: 225/45R17 is a section width of 225 mm, an aspect ratio of 45%, and a 17-inch rim. The aspect ratio is a percentage of the width rather than a measurement of its own, which is why a wider tire with the same number has a taller sidewall.
2.The sidewall height is width times aspect over 100, which gives millimetres. Convert that to inches, double it because the tire has a sidewall above the rim and below it, and add the rim diameter.
3.Keep the difference inside about 3%. Beyond that the speedometer, the odometer and — on a modern car — the traction and ABS calibration are all reading a wheel that turns at the wrong rate.

The formula

Current sidewall heightwidth_a * aspect_a / 100 / 25.4
Current overall diameterrim_a + 2 * sidewall_a
New sidewall heightwidth_b * aspect_b / 100 / 25.4
New overall diameterrim_b + 2 * sidewall_b
Difference in diameter(diameter_b - diameter_a) / diameter_a * 100
New rolling circumferencepi * diameter_b
Current revolutions per mile63360 / (pi * diameter_a)
New revolutions per mile63360 / circumference_b
True speed when the dial reads thatspeed * diameter_b / diameter_a

Source: 49 CFR 571.109 — New pneumatic tires: section width, aspect ratio and rim size, 49 CFR 571.139 — New pneumatic radial tires for light vehicles

Questions people actually ask

Why is the middle number a percentage?: Because it is a ratio, not a length. A 45 on a 225 tire is a sidewall of 101 mm; the same 45 on a 275 tire is 124 mm. That is why swapping to a wider tire without lowering the aspect ratio makes the whole wheel taller, and why plus-sizing pairs a wider tire with a smaller second number.
How far can the diameter change before it matters?: About 3% is the figure the trade works to, and it is a practical limit rather than a legal one. Past it the speedometer error becomes noticeable, the odometer undercounts or overcounts mileage, and systems that infer wheel slip from rotation speed are working from a wrong assumption.
Does a bigger tire make the speedometer read high or low?: Low. A taller tire covers more ground per turn, so the car is going faster than the dial says — the reading is behind reality. A smaller tire does the opposite and flatters you. The figure above shows the true speed for whatever the dial reads.
Why are the tire in millimetres and the rim in inches?: Because the code is a hybrid that nobody redesigned. Section width and sidewall are metric; rim diameter stayed in inches because rims were standardised first and the wheels did not change when tire labelling did. Every metric tire size carries both systems in one string, and the bridge between them is the inch, which is a defined length in millimetres rather than a measured one.

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