The diameter is the number, and the percentage follows from it.
By David Liu, founder of DIY9lives. Last updated November 2, 2026.
A tire size is a set of measurements written in code, and the code contains everything needed to work out how tall the tire is. Once you know the height, the effect on your speedometer is one division. This guide reads a size by hand, then compares two sizes the way the tire size calculator does.
Take 205/55R16. The first number is the section width in millimetres: the tire is 205 mm across at its widest point. The second is the aspect ratio, a percentage: the sidewall height is 55% of the width. The letter is the construction, radial in almost every passenger car. The last number is the wheel diameter in inches, which is the one measurement in a different unit from the rest.
That mixture of millimetres and inches is the whole complication. Convert the wheel to millimetres and the rest is arithmetic.
Sidewall height is 55% of 205 mm, which is 112.75 mm. The tire has two sidewalls — top and bottom — so that contributes 225.5 mm. The wheel is 16 inches, which is 406.4 mm. Add them:
406.4 + 225.5 = 631.9 mm, or about 24.88 inches.
| Size | Sidewall | Diameter | Circumference |
|---|---|---|---|
| 205/55R16 | 112.8 mm | 631.9 mm (24.88 in) | 1,985 mm |
| 215/45R17 | 96.8 mm | 636.6 mm (25.06 in) | 2,000 mm |
| 225/40R18 | 90.0 mm | 637.2 mm (25.09 in) | 2,002 mm |
Note what happens when the wheel grows: the aspect ratio has to fall to keep the overall diameter roughly the same. That is why plus-sizing works, and why a bigger wheel is not automatically a taller tire.
The speedometer is calibrated to the original diameter. Change the diameter and every reading is wrong by the same percentage, because the wheel covers a different distance per turn.
The error is the new diameter divided by the old, minus one. Going from 631.9 mm to 636.6 mm is 0.74% larger, so at an indicated 60 mph you are actually travelling about 60.4 mph. Going the other way to a 620 mm tire is 1.9% smaller, so an indicated 60 is an actual 58.9.
A percentage is easier to reason about than a single speed. At 2% error, every 50 miles on the odometer is really 51, and a 300-mile tank of fuel is 306. Warranty mileage, lease excess-mileage charges and resale all run off the odometer, which is why a large change is worth avoiding rather than merely noting.
A widely used rule in the tire trade is to keep the new diameter within about 3% of the original. That is a guideline rather than a legal limit, and the reasoning behind it is practical: beyond a few percent, the gearing, the anti-lock braking calibration and the speedometer are all operating outside what the car was designed for.
Two other things change with size even when the diameter is held. A wider tire changes the contact patch and can change steering feel and aquaplaning behaviour. A lower sidewall is stiffer, which sharpens response and worsens ride. Both are real, and neither shows up in the diameter arithmetic.
Load index and speed rating matter as much as the size. Fitting a tire rated below the car's requirement is a safety decision, not a style one, and the placard on the driver's door jamb states what the car needs. Do not go below it.
A larger diameter reduces the gap to the strut and the inner arch at full lock. Measure before buying, especially on a car with a lowered suspension or an unusual offset.
Sidewall height is the suspension's first spring. Losing 16 mm of it, as in the 205/55R16 to 215/45R17 comparison, is noticeable on broken pavement.
Many cars read a little high from the factory, so a slightly larger tire can bring the reading closer to the truth. That is a happy accident rather than a reason to choose a size, and it is worth checking against a GPS before you rely on it.
A different diameter spare is a temporary measure only. Tire pressure sensors are matched to wheel position in many cars and need to be re-learned or replaced when wheels are swapped.
Mismatched diameters on an all-wheel-drive car can damage the drivetrain, which is the most expensive mistake in this whole subject. On a two-wheel-drive car, keep the axle pairs matched.
For the pair in the table above, the answer is simple: 6 mm of diameter, 0.74% of speed, and a reading that is a fraction of a mile per hour out at highway speed. That is why plus-sizing one inch with the right aspect ratio is so common — the arithmetic barely notices.
The sticker on the driver's door jamb states the size, load index and speed rating the car was engineered for, plus the recommended pressures. That is the authority, not the tire that happens to be fitted. If a previous owner changed the size, the placard tells you what the car wants and the calculator tells you what the change cost.
Load index is a number that maps to a maximum load per tire, and speed rating is a letter that maps to a maximum sustained speed. Fitting a lower rating than the placard specifies reduces the car's designed margin, and insurers and inspectors in some jurisdictions care. Fitting a higher rating is harmless but can cost more and ride firmer, because a higher speed rating usually means a stiffer construction.
Pressure follows the car, not the tire. The number on the sidewall is a maximum for that tire at full load, not a recommendation. Inflating to the sidewall figure is a common mistake that produces a hard ride, uneven wear and less grip.
One more check while the wheels are off: if the car is all-wheel drive, all four tires need to be within a small circumference of each other. A single new tire on an AWD car with three worn ones can put continuous strain on the differential, which is a four-figure repair caused by a two-figure saving.
Anyone running a winter set owns two diameters. That is fine as long as the pair is consistent, but it is worth checking that the winter size does not shift the speedometer by several percent, because the error applies for the months you are most likely to be watching your speed in bad conditions.
A common winter practice is to go down a wheel size and up an aspect ratio, which keeps the diameter close and puts more sidewall between the rim and the salt. The arithmetic is the same as any other size change: work out both diameters and compare the percentage.
If you swap wheels yourself, note the sizes inside the garage door. The next person who needs that information is usually you, in November, in the cold.
Last updated: November 2, 2026. Estimates only. Not tax, legal, financial, or professional advice. See the terms.