Tire Size Calculator
Tire size calculator comparison: diameter, circumference, speedometer error and ride height between two tire sizes.
Tire 1 (original)
Tire 2 (new)
Side-by-side (scaled to diameter)
Per-tire measurements
| Tire 1 | Tire 2 |
|---|
Comparison
Speedometer: indicated vs. actual speed
| Indicated (mph) | Actual (mph) |
|---|
How the tire size calculator works
Tire sizes follow a standard format: width/aspect R diameter, for example 265/70R17 means a 265 mm wide tire with a sidewall height equal to 70% of that width, mounted on a 17" rim. To compare two sizes, the calculator first converts each to an overall diameter, then derives circumference, revolutions per mile, and how that difference affects your speedometer and ride height.
Sidewall height (mm) = width (mm) × aspect ratio ÷ 100. For 265/70R17: 265 × 0.70 = 185.5 mm, or 7.303".
Overall diameter = rim diameter + 2 × sidewall height. For 265/70R17: 17" + 2 × 7.303" = 31.61". That matches the commonly cited ~31.6" diameter for this size.
Circumference = diameter × π. Revolutions per mile = 63,360 ÷ circumference (in inches), since a mile is 63,360 inches.
Speedometer error happens because your speedometer counts wheel revolutions and assumes the original tire's circumference. If you fit a larger tire, each revolution covers more ground than the speedometer expects, so your actual speed is higher than the indicated speed: actual = indicated × (new diameter ÷ original diameter). A smaller tire has the opposite effect.
Ride height change is half the diameter difference, since the extra (or reduced) diameter is split between the top and bottom of the tire around the fixed wheel center. A rule of thumb used by many shops is to stay within about 3% of the stock diameter to avoid speedometer error, fender rubbing, and extra strain on the drivetrain; this tool flags comparisons that exceed that threshold.
Tire Size Calculator: frequently asked questions
How is overall tire diameter calculated?
Sidewall height (mm) = width (mm) x aspect ratio / 100. Convert that to inches, double it (top and bottom sidewall), and add the wheel diameter: overall diameter = rim diameter + 2 x sidewall height. For example a 265/70R17 has a 7.30" sidewall, giving 17 + 2 x 7.30 = 31.6" overall diameter.
How much tire size difference is safe?
Most mechanics and tire shops recommend staying within 3% of the original overall diameter to avoid speedometer error, clearance rubbing and drivetrain stress. This tool flags any comparison over that 3% threshold.
Why does my speedometer read wrong after changing tire size?
Your speedometer is calibrated to your original tire's revolutions per mile. Larger tires cover more distance per revolution, so the speedometer under-reads your actual speed; smaller tires make it over-read.
What is the difference between tire diameter and circumference?
Diameter is the straight-line height of the inflated tire. Circumference is the distance the tire covers in one full revolution (diameter x pi). Circumference is what determines revolutions per mile and speedometer accuracy.
Does plus-sizing a tire change my ride height?
Yes. Because the wheel diameter portion stays fixed to the rim, any change in overall tire diameter is split evenly between the top and bottom, so half of the diameter difference shows up as a ride height change at each corner.