Decoding a tire size
A code such as 225/45R17 packs three measurements together. The first number is the section width in millimetres (how wide the tread sits). The second is the aspect ratio — the sidewall height as a percentage of that width — so a 45 on a 225 tire means a sidewall 101 mm tall. The R marks radial construction, and the last number is the wheel (rim) diameter in inches.
From those, the overall diameter is the rim plus two sidewalls (top and bottom), converted to inches. Multiplying the diameter by π gives the rolling circumference, and dividing a mile’s worth of inches by that circumference gives revolutions per mile.
Why overall diameter matters
Overall diameter quietly affects several things you feel on the road.
- A taller tire raises ride height and ground clearance slightly.
- It lowers effective gearing, which can blunt acceleration but ease highway revs.
- It changes how many times the wheel turns per mile, which is what drives speedometer and odometer accuracy.
- Too large a change can rub against bodywork or suspension at full lock or compression.
Reading the comparison
Turn on the second size to see how a swap changes things. The diameter difference shows the percentage change in overall height, and the chart puts the two diameters side by side so the gap is easy to judge. Aim to keep a replacement within roughly 3% of the original diameter to avoid throwing off the speedometer, gearing and clearances.
How tire size fools the speedometer
A speedometer is calibrated for the original tire’s rolling distance. Fit a larger-diameter tire and the wheel turns fewer times for the same distance, so the gauge reads slower than your true speed; a smaller tire reads faster. The speedometer-error figure estimates the gap at an indicated 60 mph. This is a geometric estimate from nominal sizes — real rolling diameter varies with load, pressure and wear.
Formula
sidewall = width·aspect/100; diameter = rim + 2·sidewall/25.4; circumference = π·diameterFrequently asked questions
- How does tire size affect my speedometer?
- A larger overall diameter means the wheel turns fewer times per mile, so the speedometer reads lower than your true speed (and vice-versa).