Compare Tire Size Calculator
Put two sidewall codes side by side and read what actually changes between them: overall diameter in inches and as a percentage, sidewall height, circumference and revolutions per mile for each size, plus what the speedometer reads at a true 60 mph on the new one and the miles the odometer gains or loses over 100 driven. Both sides are exact arithmetic on the codes, with no catalogue behind them and no verdict about your car.
- Section width
- 225 mm (8.86 in)
- Sidewall (45%)
- 3.99 in (101.3 mm)
- Rim diameter
- 17 in (431.8 mm)
- Overall diameter
- 24.97 in (634.3 mm)
- Circumference
- 78.45 in (1992.7 mm)
- Revolutions per mile
- 808
- Section width
- 235 mm (9.25 in)
- Sidewall (40%)
- 3.70 in (94.0 mm)
- Rim diameter
- 18 in (457.2 mm)
- Overall diameter
- 25.40 in (645.2 mm)
- Circumference
- 79.80 in (2027.0 mm)
- Revolutions per mile
- 794
Two codes side by side, with the diameter difference and what it does to the speedometer.
What the change does
The speedometer and the odometer are out by the same percentage as the diameter, at every speed and in any units: a taller tire turns fewer times per mile, so the needle and the mileage both read low.
Common questions
- How do I compare two tire sizes?
- Type the size the car is on now in the first field and the size you are considering in the second. The two panes show every derived dimension for each, and the strip under the new size carries the diameter difference as a percentage. Sizes can be written any way the sidewall does: 225/45R17, 225/45 R17, P225/45R17 or 225/45r17 all read the same, and Swap turns the comparison around.
- What does going up a size actually change?
- Three things you can measure: the overall diameter, which changes ride height and gearing; the circumference, which changes revolutions per mile; and the section width, which changes how much rubber is across the wheel. A wider tire on a lower profile can come out shorter than the tire it replaces, which is why the percentage matters more than the numbers looking bigger. 225/45R17 to 235/40R18 is wider and a larger rim, and still only 1.72 percent taller.
- Does the speedometer error work the same in kilometres per hour?
- Yes. The error is a ratio of diameters, so it is the same percentage at every speed and in every unit. A size that reads 58.99 at a true 60 mph reads 98.31 at a true 100 km/h. The page states the percentage as well as the reading at 60 so you can apply it to whatever your dash shows.
- Which size should I put in which field?
- The size on the car now goes in the current field, because the speedometer and the odometer were calibrated for it. That order is what makes the reading at 60 and the odometer figure mean anything: they are what the instruments will do once the new size is fitted. Putting the same code in both fields gives a zero difference and a speedometer reading of exactly 60, which is a quick way to check you have typed what you meant.
- Will a difference this small cause a problem?
- That is a question about your car rather than about arithmetic, and this page will not answer it. Clearance, suspension travel and how the electronics were calibrated decide it, and none of them can be seen from a sidewall code. The comparison gives you the exact figure to take to your vehicle's documentation or to a fitter.
Exact arithmetic on the sidewall code, which is a definition rather than an estimate. It cannot know your wheel well's clearance, whether a load or speed rating suits your vehicle, or the true mounted diameter, which changes with pressure, load and wear.