Tire Size Calculator
Compare two tire sizes' overall diameter, circumference, and revolutions per mile — and see how much your speedometer would drift if you switch.
Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.
Diameter Difference
+2.05%
Indicated 60 mph Is Actually
61.23 mph
Example
Tire 1 has an overall diameter of 28.52 in, while Tire 2 is 2.05% larger at 29.1 in. If your speedometer is calibrated for Tire 1 and you switch to Tire 2, an indicated 60 mph would actually be about 61.2 mph.
Tire 2 vs. Tire 1 — percent difference by metric
What is a Tire Size Calculator?
A tire size calculator translates the numbers printed on a tire's sidewall — width, aspect ratio, and rim diameter — into real-world measurements like overall diameter and circumference. It's most useful for comparing two tire sizes before switching, since even a tire that "looks close" on paper can meaningfully change your speedometer accuracy, fuel economy, and ground clearance.
Tire sizes are written in a standard format like 225/65R17: 225 is the section width in millimeters, 65 is the aspect ratio (sidewall height as a percent of width), and 17 is the rim diameter in inches.
Side-by-Side Comparison
| Metric | Tire 1 (Current) | Tire 2 (New) |
|---|---|---|
| Sidewall Height | 146.25 mm | 141 mm |
| Overall Diameter | 28.516 in | 29.102 in |
| Circumference | 89.585 in | 91.428 in |
| Revolutions per Mile | 707.3 | 693.0 |
How These Metrics Are Calculated
The aspect ratio is a percentage of the width, not a fixed number — so the same "65" aspect ratio produces a taller sidewall on a wider tire. The rim diameter is added because the tire wraps entirely around the wheel; the overall diameter includes the wheel plus both the top and bottom sidewall.
Why Speedometer Accuracy Changes
Your speedometer is calibrated assuming a specific tire circumference and counts wheel rotations to estimate speed. A larger-diameter tire covers more ground per rotation, so the same number of rotations per minute now corresponds to a higher actual speed than the speedometer displays — meaning you're going faster than indicated. A smaller-diameter tire has the opposite effect.
The "Plus Sizing" Rule of Thumb
Many enthusiasts follow a rough rule when upsizing wheels: keep the overall tire diameter within about 3% of the original to avoid meaningfully affecting speedometer accuracy, anti-lock braking calibration, and ground clearance. Going up in rim diameter usually means dropping to a lower-profile (smaller aspect ratio) tire to keep the overall diameter close to the original.
Example — Your Current Inputs
Tire 1 has an overall diameter of 28.52 in, while Tire 2 is 2.05% larger at 29.1 in. If your speedometer is calibrated for Tire 1 and you switch to Tire 2, an indicated 60 mph would actually be about 61.2 mph.
Additional Example — Same Diameter, Bigger Wheel
A 215/65R16 tire has roughly the same overall diameter as a 225/55R17 — the wider, lower-profile tire on the bigger 17-inch wheel compensates for its shorter sidewall, keeping the speedometer essentially unaffected despite the different-looking size on paper.
About These Parameters
- Width (mm)
- The tire's section width in millimeters — the first number in a size like 225/65R17.
- Aspect Ratio (%)
- The sidewall height as a percentage of the width. Lower numbers mean a shorter, stiffer "low-profile" sidewall; higher numbers mean a taller, more cushioned sidewall.
- Rim Diameter (in)
- The diameter of the wheel the tire mounts on, in inches — the number after the "R" in a size like 225/65R17.
Frequently Asked Questions
How much tire size difference is considered safe?
Many tire professionals recommend staying within about 3% of the original overall diameter to avoid meaningfully affecting speedometer accuracy, traction control, and fender clearance.
Why does a bigger tire make my speedometer read low?
A larger tire covers more distance per revolution, so it needs fewer revolutions to travel a mile. Since your speedometer estimates speed from revolutions, it undercounts your actual speed — you're going faster than it shows.
Does tire size affect fuel economy?
Yes, in a couple of ways — larger, heavier tires increase rolling resistance and rotational mass, which can reduce fuel economy, while a taller overall diameter can slightly improve highway fuel economy by lowering engine RPM at a given speed.
Can I mix different tire sizes on the same car?
It's generally not recommended except on vehicles specifically designed for staggered fitments (often performance cars with different front/rear sizes) — mismatched diameters front-to-back or side-to-side can affect traction control, all-wheel-drive systems, and handling balance.