Tire Size Calculator

What the tire size calculator does

That string of numbers on the side of your tire, something like 205/55 R16, actually tells you the tire's real shape, once you know how to read it. This tool reads it for you. Enter the size and it works out the things that string hides: the sidewall height, the overall diameter, the circumference, and how many times the tire turns in a mile or a kilometer.

It also has a compare mode. Switch it on, enter a second size, and it shows you exactly how the two differ, which is the part that matters when you are thinking about fitting a different tire.

Reading the numbers on the sidewall

Take 205/55 R16 as the example. The first number, 205, is the tire's width in millimeters. The second, 55, is the aspect ratio, which is the sidewall height written as a percentage of the width, so this sidewall is 55 percent of 205. The R means radial construction, and the last number, 16, is the wheel diameter in inches that the tire fits. So the size is a mix of millimeters, a percentage, and inches all in one code, which is exactly why a tool that untangles it is handy.

How to use it

1. Choose whether to compare. Leave it on No to look at a single tire, or switch it to Yes to compare two.
2. Enter the size. The width, then the aspect ratio, then the wheel diameter after the R, reading straight off your sidewall.
3. Enter the second size if you turned comparison on.

Press Calculate for the breakdown, or Reset to clear the boxes.

How the dimensions are worked out

It starts with the sidewall, since that is what the aspect ratio gives you:

Sidewall height = width × aspect ratio ÷ 100

The overall diameter is then the wheel diameter plus two sidewalls, one at the top and one at the bottom, with the units lined up between millimeters and inches. Multiply the diameter by pi for the circumference, the distance the tire covers in one full turn. And once you have the circumference, the revolutions per mile is simply how many of those turns fit into a mile, which is 63,360 inches:

Revolutions per mile = 63,360 ÷ circumference in inches

The revolutions per kilometer come from that same figure, adjusted for the shorter distance.

An example with real numbers

Take that 205/55 R16.

• Sidewall height = 205 × 55 ÷ 100 = 112.75 mm, which is about 4.4 inches
• Overall diameter = 16 inch wheel plus two sidewalls = about 24.9 inches
• Circumference = pi × 24.9 = about 78.2 inches
• Revolutions per mile = 63,360 ÷ 78.2 = about 811

So that everyday-looking tire stands about 24.9 inches tall and turns roughly 811 times for every mile you drive. None of that is printed on the sidewall, but all of it is hiding inside those three numbers.

Comparing two sizes and your speedometer

This is where the compare mode earns its keep. Your speedometer and odometer are calibrated for the tire diameter your car came with. Fit a tire with a different overall diameter and both readings drift, because the distance covered per turn has changed. A taller tire makes your speedometer read low, so you are actually going faster than it says, and a shorter tire does the opposite.

Say you go from that 205/55 R16, at about 24.9 inches, up to a 215/60 R16, which comes out around 26.2 inches, roughly 5 percent taller. At an indicated 60 mph you would really be doing about 63. The compare mode shows you the percentage difference in diameter and in revolutions per mile, which is precisely how far off your gauges will read. As a rule, keeping the overall diameter within about 3 percent of the original is the usual advice for staying close to accurate, which is the whole idea behind a sensible plus-size.

Questions people ask

What do the numbers in a tire size mean?

In a size like 205/55 R16, 205 is the width in millimeters, 55 is the sidewall as a percentage of that width, R is radial construction, and 16 is the wheel diameter in inches.

Will a different tire size affect my speedometer?

Yes, if the overall diameter changes. A taller tire makes the speedometer read low and the odometer undercount, a shorter tire does the reverse. Keeping the new diameter within about 3 percent of the original keeps the error small.

What is the aspect ratio?

It is the sidewall height expressed as a percentage of the tire width. A 55 means the sidewall is 55 percent of the width. A lower number is a shorter, lower-profile sidewall.

What are revolutions per mile good for?

They tell you how many times the tire turns per mile, which is what the speedometer and odometer count. Comparing this figure between two sizes shows exactly how much your readings would shift.

References

1. National Institute of Standards and Technology (NIST), Special Publication 811: Guide for the Use of the International System of Units (SI), for the inch and millimeter relationships and 63,360 inches per mile. https://www.nist.gov/pml/special-publication-811