Tire Size

Comparing Tire Diameter From the Size Code

A tire’s size code (like 225/50R17) encodes its width, sidewall height, and rim size — but not its overall diameter, which is what actually determines how a tire swap affects your speedometer and ride height. Enter an original tire size and a replacement size to see exactly how much the overall diameter changes.

The three numbers in a tire size mean:

• 225 — section width in millimeters (how wide the tire is).
• 50 — aspect ratio, the sidewall height as a percentage of the width (not a fixed number).
• 17 — rim diameter in inches (the wheel it mounts on).

Because the sidewall height is a percentage of width, two tires with the same aspect ratio number can have very different actual sidewall heights if their widths differ — which is exactly why a “same-looking” size change can still shift the overall diameter more than expected.

The Formula

1. Sidewall height (mm) = $\vD{\text{Sidewall Height}} = \vA{\text{Width}} \times \dfrac{\vB{\text{Aspect Ratio}}}{100}$.
2. Overall diameter (mm) = $\vE{\text{Overall Diameter}} = \vC{\text{Rim Diameter}} + 2 \times \vD{\text{Sidewall Height}}$ — the sidewall appears twice, once above the rim and once below.
3. Circumference = $\vF{\text{Circumference}} = \vE{\text{Overall Diameter}} \times \pi$, and revolutions per mile = $\dfrac{63,360}{\vF{\text{Circumference}}}$ (there are 63,360 inches in a mile).
4. Since a car’s speedometer and odometer are calibrated to the original tire’s revolutions per mile, a replacement tire with a different diameter makes the car travel a different real distance per wheel revolution than the speedometer assumes — throwing off both readings by roughly the same percentage as the diameter change.

Worked Example

Comparing a 225/50R17 (original) to a 235/45R18 (a common “plus-one” wheel upgrade):

The new tire is about 1.8% larger in overall diameter — small enough to fall within the ±3% tolerance many tire shops use as a rule of thumb for a safe fitment, but still enough to make the speedometer read about 1.8% slower than the car’s true speed.

Key Factors to Consider

• A speedometer/odometer offset from a tire size change is directional, not just a magnitude issue. A larger-diameter replacement tire makes the actual speed and distance traveled HIGHER than what the speedometer/odometer shows (since each wheel revolution covers more real ground than the calibration assumes), while a smaller-diameter tire makes them read LOWER than reality — knowing which direction the error runs matters for anyone relying on the speedometer near a speed limit.
• Ground clearance and other mechanical clearances (fender wells, suspension travel) can be affected by a tire diameter change beyond just speedometer accuracy. A significantly larger tire can rub against the fender or suspension components under certain conditions (like full steering lock or suspension compression) — this is a real fitment concern separate from the pure diameter-percentage math this calculator computes.
• Load rating and speed rating are separate codes on a tire that this diameter comparison doesn’t address. A replacement tire should match or exceed the original’s load-carrying capacity and speed rating, not just have a similar overall diameter — these ratings are printed on the tire’s sidewall alongside the size code covered here.
• Some vehicles with electronic stability control or all-wheel-drive systems are more sensitive to tire diameter mismatches than others. A meaningful diameter difference between tires on the same axle (or across axles on some AWD systems) can trigger warning lights or, in some cases, drivetrain stress — checking a vehicle manufacturer’s own tolerance guidance is worth doing for these vehicle types specifically.

Common Mistakes

• Comparing aspect ratio numbers directly instead of computing actual sidewall height. The same aspect ratio (like 50) produces a different sidewall height in millimeters on a wider tire than on a narrower one, since it’s a percentage of width — two tires with matching aspect ratios can still have a meaningfully different overall diameter.
• Assuming any size that “fits on the rim” is a safe swap. A tire can physically mount on a given rim diameter while still producing a diameter change well outside the commonly-cited ±3% tolerance — rim compatibility and overall-diameter compatibility are two separate checks.
• Overlooking load rating and speed rating while focused on diameter. A replacement tire that matches the original’s diameter closely can still be a poor or unsafe substitute if its load or speed rating doesn’t meet the original’s — diameter is only one part of a full tire comparison.

Useful to Know

• Wondering how a tire size change affects real-world fuel economy, not just the speedometer? Gas Mileage (MPG) Calculator estimates fuel efficiency from actual driving data.
• Curious what a size change might mean for your car’s actual performance? Horsepower Calculator estimates output from drag-strip performance data.