Investigate differences between tire-size calculator results

The short answer

Tire size calculators usually disagree for one of five reasons: they were given slightly different inputs, they use different formulas or size conventions, they convert between millimeters and inches differently, they round at different points, or they report different kinds of dimensions. Some calculate nominal geometry straight from the size code. Others show a manufacturer's published figure measured on a specific rim. When two tools differ by a few hundredths of an inch, rounding is the usual cause. When they differ by a tenth of an inch or more, or when speedometer results point in opposite directions, the cause is more likely a different method, a different conversion or a different definition of the output.

Some terms used throughout this guide:
Nominal dimensions are the values calculated purely from the size code.
Measured dimensions are what a specific tire actually measures when mounted and inflated on a stated rim.
The aspect ratio is the sidewall height expressed as a percentage of the section width.

How a calculator turns a size code into inches

A metric size such as 205/55R16 has three parts:
205 is the section width in millimeters.
55 is the aspect ratio, so the sidewall is 55 percent of 205 mm.
16 is the rim diameter in inches.
The code mixes metric and imperial units, so every calculator has to convert one into the other. The overall diameter is the rim diameter plus two sidewall heights, because there is one sidewall above the wheel and one below it. Circumference and revolutions per mile are then calculated from that diameter.

Sidewall height     = 205 mm x 0.55          = 112.75 mm
Rim diameter        = 16 in x 25.4 mm/in     = 406.4 mm
Overall diameter    = 406.4 + 2 x 112.75     = 631.9 mm
                    = 631.9 / 25.4           = 24.88 in
Circumference       = pi x 24.88 in          = 78.16 in
Revolutions per mile = 63,360 in / 78.16 in  = about 811

These figures describe the ideal, unloaded geometry that the size code implies. They are not a measurement of any particular tire. A real tire's diameter depends on:
The manufacturer's design.
The width of the rim it is mounted on.
Inflation pressure.
Tread depth.
The load on the tire. Under the vehicle's weight, the effective rolling radius is smaller than the unloaded radius, so a tire rolling on the road turns more times per mile than the simple formula predicts.

Where the numbers start to diverge

Different inputs or size conventions

The most common cause of a mismatch is that the two calculators were not given identical inputs. Typical examples:
A mistyped aspect ratio.
A different rim size selected from a dropdown.
A tool that silently defaults to a size you did not choose.
Flotation sizes, often used on trucks and off-road vehicles, follow a different convention. In 33x12.50R15, the first number is a nominal overall diameter in inches, not a width in millimeters. One calculator may report 33.0 inches directly from the code. Another may report the manufacturer's measured diameter for a specific tire, which often comes out smaller than the nominal figure. Both answers can be correct for what they claim to describe.

Conversion constants

One inch is exactly 25.4 mm. A tool that uses a rough 25 mm per inch, or that converts the width to inches and rounds it early, will drift from the correct result. In the illustrative example below, each line is the same 205/55R16 calculation done a different way:
Width converted at full precision (8.07 in): 24.88 in overall diameter.
Width rounded to 8.1 in before the sidewall is calculated: 24.91 in.
Width converted with 25 mm per inch (8.2 in): 25.02 in, which is wrong by about 0.14 in.

Intermediate and output rounding

Rounding partway through a calculation changes the answer, even when the formula is correct. Here is a hypothetical example using the same 205/55R16 math. Rounding the sidewall from 112.75 mm to 113 mm raises the diameter from 631.9 mm to 632.4 mm. That is roughly 24.88 in versus 24.90 in.

Output rounding causes similar differences. One tool may show 24.88 in, another 24.9 in, and a third 25 in, all from the same underlying value. Percentage comparisons are affected too. Comparing 205/55R16 with 225/50R17 at full precision gives a diameter increase of about 3.94 percent. If both diameters are first rounded to one decimal place (24.9 in and 25.9 in), the increase becomes about 4.02 percent.

Nominal versus published dimensions

Many calculators use only the formula, so they return the same answer for every brand and model of a given size. Published manufacturer specifications are different. They list a specific tire's overall diameter, section width and revolutions per mile, measured on a stated measuring rim under defined conditions. Two tires with the same size code can therefore have published diameters that differ from each other and from the formula.

Section width is especially sensitive to rim width. A calculator that reports width without stating which rim it assumes is giving you a nominal figure, not what you will see on your wheel.

Speedometer and revolutions-per-mile conventions

Speedometer results often look contradictory because tools answer different questions. Take the 3.94 percent larger tire from the example above, on a vehicle whose speedometer was accurate with the original size:
One calculator answers: when the speedometer shows 60 mph, how fast are you actually going? About 62.4 mph.
Another answers: when you are actually going 60 mph, what does the speedometer show? About 57.7 mph.
Both figures come from the same diameter ratio. They are simply the inverse of each other.

Revolutions per mile also vary by method. A formula-based tool gives about 780 for 225/50R17 from its unloaded circumference. A manufacturer's figure measured under load will usually be higher, because a loaded tire rolls on a smaller effective radius. Some calculators also use a rounded value of pi, such as 3.14. This changes circumference slightly, by a few hundredths of an inch.

Quick reference: cause, effect and how to spot it

Cause	What it changes	How to spot it
Input or convention mismatch	Any or all outputs, sometimes by a large amount	Check the exact size string, rim size and whether the size is metric or flotation
Conversion constant	Inch values, especially width and diameter	Recompute using 25.4 mm per inch and compare
Intermediate rounding	Small shifts in diameter and percentages	Recompute at full precision; differences are usually a few hundredths of an inch
Output rounding	Displayed digits only	Compare the number of decimal places shown
Nominal vs published basis	Diameter, width, revolutions per mile	Look for a brand, model, measuring rim or the word "measured"
Speedometer direction	Speedometer figures appear opposite	Check whether the tool shows actual speed or indicated speed

How to audit two calculators that disagree

• 1. Enter identical inputs in both tools. Type the full size string exactly, confirm the rim diameter, and set the same units. Write down exactly what you entered.
• 2. Note what each tool says it is calculating. Check whether it uses a formula, a manufacturer specification or both, and whether it names a tire, a rim width or measurement conditions. If a tool does not explain its method, record the method as unknown rather than guessing.
• 3. Recompute the nominal values yourself at full precision. Use 25.4 mm per inch and do not round until the end. Use the calculation shown earlier in this guide as a template.
• 4. Apply each tool's displayed rounding to your full-precision result. If that reproduces the tool's number, the difference was only rounding.
• 5. For any spec-based result, record the exact tire model, the date or revision of the specification, the measuring rim and the conditions. Keep nominal and measured figures in separate columns.
• 6. For speedometer outputs, decide which question each tool answers: actual speed at a given indicated speed, or indicated speed at a given actual speed. Convert one to the other before comparing.
• 7. Classify whatever difference remains as input, method, conversion, rounding or specification basis.

A simple worksheet makes this easier. Use one row per calculator and add columns for:
Tool and version.
Exact inputs.
Units.
Stated formula.
Conversion constant.
Rounding behavior.
Dimension basis (nominal or measured).
Any specification cited.
The resulting numbers.

What matching results do and don't prove

When two calculators agree, it only means their arithmetic and assumptions line up. It does not show that a tire will fit your vehicle. Fitment depends on things these tools do not model:
Wheel offset and width.
Suspension and body clearance at full steering lock and full compression.
Load index and speed rating.
The vehicle maker's approved sizes.
For a fitment decision, confirm against the vehicle's tire placard or owner's manual, the specific tire's published specifications, and a qualified tire professional.