Tire sizes are written in three numbers that mix two unit systems, which is why comparing two of them in your head does not work. Width is millimetres, the aspect ratio is a percentage of that width, and the wheel is inches.
Reading the size
In 225/60R16, the 225 is the section width in millimetres. The 60 is the aspect ratio — the sidewall height is 60 percent of 225, which is 135 mm or 5.31 inches. The 16 is the wheel diameter in inches.
Overall diameter is the wheel plus two sidewalls, because there is one above and one below the wheel. So 16 plus 10.63 gives 26.63 inches.
That is the number that matters. Everything else — speedometer, gearing, clearance — follows from overall diameter, and it is the number the size does not tell you directly.
Why the same aspect ratio means different heights
The aspect ratio is a percentage of the width, not an absolute measurement. So a 245/45 has a taller sidewall than a 225/45, even though the ratio is the same — 45 percent of 245 is more than 45 percent of 225.
This catches people going wider. Fitting a wider tire at the same aspect ratio and the same wheel makes the tire taller as well as wider, which affects clearance in a wheel well that may not have room.
Going wider usually means dropping the aspect ratio to keep the diameter similar, which is the pattern in the plus-sizing tables.
Speedometer and odometer error
A speedometer counts wheel revolutions and multiplies by an assumed circumference. Fit a taller tire and each revolution covers more ground, so you are actually going faster than the dial says.
This calculator estimates the change from the nominal size geometry. Actual rolling circumference varies with tire model, load, pressure and tread wear, so a measured or manufacturer-published revolutions-per-mile figure is more precise.
Some cars can be recalibrated in software, some need a module, and some cannot be changed. On a modern car, worth checking before buying tires.
The three percent rule
A three percent diameter difference is a common preliminary comparison, not a safety standard or fitment approval.
Even inside that range, the tire must match the vehicle’s load and speed requirements, fit an approved rim width, clear the body and suspension through their full travel, and work with the vehicle’s tire-pressure and stability systems.
Use the tire and loading label and owner’s manual as the starting point. Ask the vehicle or tire manufacturer about substitutions and recalibration.
What this calculator will not tell you
It will not tell you whether a tire fits your car. That depends on wheel well clearance, suspension travel, steering lock, wheel offset and brake caliper geometry — none of which can be worked out from a tire size.
It will not tell you a load or speed rating. Those are letters on the sidewall and they are a safety matter: a tire rated below what the vehicle needs is not a substitution, it is a hazard.
And it will not tell you a pressure. The correct pressure is on the sticker in the door frame, for the size the car came with — changing size can change that, and it is a question for a tire shop rather than a calculator.