Tire size calculator

What is a tire size calculator?

A tire size calculator turns the short code printed on a tire sidewall — something like 275/60R20 — into the real dimensions the tire actually has: how tall its sidewall is, how big the whole wheel-and-tire package measures across, and how far the car travels in one rotation.

Those three numbers on the sidewall are a designation, not a measurement. They are deliberately compact, and two of them are quoted in different systems, which is exactly why the code is so hard to read at a glance:

• 275 — the section width, the widest point of the inflated tire, always written in millimeters.
• 60 — the aspect ratio, the sidewall height expressed as a percentage of the section width. It is not a length at all.
• R — radial construction (the letter carries no size information).
• 20 — the rim diameter, the wheel the tire mounts onto, always written in inches.

Because the width is metric, the sidewall is a percentage and the rim is imperial, you cannot compare two sizes by eye. 275/60R20 and 285/65R20 look almost the same, yet one stands 40 mm taller than the other. The calculator resolves all three parts into a single set of physical dimensions so the comparison becomes obvious.

Why the overall diameter matters

Almost everything that changes when you fit a different tire traces back to one number: the overall diameter.

• Speedometer and odometer accuracy. Both instruments count wheel revolutions and multiply by the rolling circumference the car was calibrated for. Fit a taller tire and each revolution covers more ground than the car assumes, so the speedometer reads low — you are going faster than it says, and the odometer under-counts your mileage.
• Effective gearing. A taller tire acts like a taller final drive: slightly lower engine revs at cruising speed, slightly less acceleration and less pulling power in the low gears.
• Clearance. Extra diameter and extra width both eat into the space between the tire and the fender lip, strut, or control arm, most obviously at full steering lock and full suspension compression.
• Driver-assistance systems. ABS, traction control and stability control all reason about wheel speed, and they inherit the same calibration error the speedometer does.

For those reasons, replacement sizes are conventionally kept within about ±3 % of the original overall diameter. The calculator reports the difference as a percentage so you can check that limit directly.

How the calculator works

Enter the three parts of the size you have now, then the three parts of the size you are considering. The results update as you type — there is no button to press.

1. Fill in the section width, aspect ratio and rim diameter of your current tire.
2. Fill in the same three numbers for the new tire.
3. Read the sidewall height, overall diameter and rolling circumference of each set. Each of those is a real physical length, so its dropdown converts freely between millimeters, centimeters, meters, inches and feet.
4. Read the revolutions per mile and per kilometer — these are counts, so both are always shown.
5. Read the comparison: the diameter difference as a percentage, and your true road speed at an indicated 60.

Formulas

The sidewall height is the aspect ratio applied to the section width, so a “60-series” tire has a sidewall 60 % as tall as the tire is wide:

$$H = \frac{W \times A}{100}$$

The overall diameter is the rim converted from inches to millimeters, plus one sidewall at the top and one at the bottom:

$$D = (R \times 25.4) + 2H$$

The rolling circumference is the distance covered in one revolution:

$$C = \pi \times D$$

Revolutions per unit of distance follow directly, using the number of millimeters in a mile and in a kilometer:

$$N_{mi} = \frac{1609344}{C}$$ $$N_{km} = \frac{1000000}{C}$$

Comparing two sets, the diameter difference is:

$$\Delta = \frac{D_{new} - D_{old}}{D_{old}} \times 100$$

And because the speedometer is still calibrated for the original circumference, the true speed when it indicates 60 is:

$$v = 60 \times \frac{D_{new}}{D_{old}}$$

Where:

• W — section width, in millimeters (the first number of the designation)
• A — aspect ratio, in percent (the second number)
• R — rim diameter, in inches (the number after the R)
• H — sidewall height
• D — overall diameter
• C — rolling circumference
• N — revolutions per mile or per kilometer
• Δ — diameter difference, in percent
• v — true speed at an indicated 60

Worked examples

Example 1: decoding 275/60R20

A 275/60R20 tire on a 20-inch wheel. The sidewall is 60 % of 275 mm:

$$H = \frac{275 \times 60}{100} = 165$$

The rim contributes 20 × 25.4 = 508 mm, and there is a sidewall above and below it:

$$D = 508 + 2 \times 165 = 838$$

So the tire stands 838 mm tall overall. Its rolling circumference is:

$$C = \pi \times 838 = 2632.65$$

which gives:

$$N_{mi} = \frac{1609344}{2632.65} = 611.30$$ $$N_{km} = \frac{1000000}{2632.65} = 379.84$$

The tire turns about 611.30 times per mile, or 379.84 times per kilometer.

Example 2: upsizing to 285/65R20

Now the same 20-inch wheel with a 285/65R20 tire. The sidewall grows twice over — a wider tire and a taller percentage:

$$H = \frac{285 \times 65}{100} = 185.25$$ $$D = 508 + 2 \times 185.25 = 878.5$$ $$C = \pi \times 878.5 = 2759.89$$

That is 583.12 revolutions per mile and 362.33 per kilometer. Against the original 838 mm:

$$\Delta = \frac{878.5 - 838}{838} \times 100 = 4.83$$

The new tire is 4.83 % taller — beyond the usual ±3 % guideline, so clearance is worth checking. And the speedometer now under-reads:

$$v = 60 \times \frac{878.5}{838} = 62.90$$

When the dial shows 60, the car is actually doing about 62.90.

Example 3: downsizing to 265/50R20

A shorter tire on the same wheel. The sidewall is 265 × 50 / 100 = 132.5 mm, so:

$$D = 508 + 2 \times 132.5 = 773$$ $$\Delta = \frac{773 - 838}{838} \times 100 = -7.76$$ $$v = 60 \times \frac{773}{838} = 55.35$$

This size is 7.76 % shorter, and the error runs the other way: an indicated 60 is really about 55.35. The speedometer is now optimistic, which is the more expensive kind of mistake.

Example 4: a smaller wheel, 225/45R17

Not every size sits on a 20-inch rim. For 225/45R17:

$$H = \frac{225 \times 45}{100} = 101.25$$ $$D = (17 \times 25.4) + 2 \times 101.25 = 634.3$$

The overall diameter is 634.3 mm — the 17-inch rim contributes 431.8 mm and the two sidewalls add 202.5 mm.

Practical notes

• The calculated diameter is a nominal figure. A real tire is slightly shorter under load, because the loaded sidewall deflects. Manufacturers publish both a nominal diameter and a loaded radius, and their revolutions-per-mile figures come from rolling tests rather than from the formula above, so expect a difference of one or two percent against the catalog.
• Aspect ratio is a ratio, not a length. A 45-series 225 mm tire and a 45-series 275 mm tire have very different sidewall heights (101.25 mm versus 123.75 mm) despite the identical middle number.
• Width and diameter are separate constraints. A size can pass the ±3 % diameter check and still rub, because the extra section width moves the shoulder closer to the suspension. Check both.
• Keep all four corners matched. On all-wheel-drive vehicles especially, mixed rolling circumferences force the center differential or clutch pack to slip continuously, which is a genuine mechanical risk rather than a comfort issue.
• Rim width matters too. Every tire size has a range of approved rim widths. Mounting a wide tire on a narrow rim pinches the tread profile and changes the real section width away from the nominal figure.
• Load index and speed rating are not size. The characters after the size — such as 115T — describe carrying capacity and maximum speed. Replacements should meet or exceed the original ratings even when the dimensions match perfectly.

Frequently asked questions

What do the numbers on a tire mean?

In 275/60R20 the 275 is the section width in millimeters, the 60 is the aspect ratio (the sidewall is 60 % of the width), the R means radial construction, and the 20 is the rim diameter in inches.

How do I calculate the overall diameter of a tire?

Multiply the width by the aspect ratio and divide by 100 to get the sidewall height, multiply the rim diameter by 25.4 to convert it to millimeters, then add two sidewalls: 275 × 60 / 100 = 165, and 508 + 2 × 165 = 838 mm.

How much can I change tire size safely?

The usual guideline is to stay within about ±3 % of the original overall diameter. Larger changes noticeably affect the speedometer, the effective gearing and the driver-assistance calibration, and they narrow the clearance to bodywork and suspension.

Why is my speedometer wrong after fitting new tires?

The speedometer counts wheel revolutions and multiplies by the circumference the car was built around. A taller tire covers more ground per revolution, so the display reads low; a shorter tire reads high. Going from 275/60R20 to 285/65R20 makes an indicated 60 into a true 62.90.

Are two tires with the same diameter interchangeable?

Not automatically. Equal diameters keep the speedometer honest, but the tires can still differ in section width, load index, speed rating and approved rim width — all of which have to suit the vehicle as well.

Does a taller tire improve fuel economy?

Sometimes marginally, because the effective gearing drops engine revs at cruising speed, but the taller tire is usually heavier and often wider, which adds rotating mass and rolling resistance. In practice the two effects tend to cancel out.