Tire Size Calculator
Enter your current tire size and the new size you're considering — results update as you type, and the link in your address bar always reflects your numbers.
How this calculator works
A tire's overall diameter comes from three numbers molded into its sidewall: width (mm), aspect ratio (the sidewall height as a percent of width), and wheel diameter (inches). Sidewall height in millimeters is width × (aspect ÷ 100); converted to inches and doubled (one sidewall on top, one on the bottom) and added to the wheel diameter, that gives the tire's overall rolling diameter. Comparing that number for your current tire (A) and a proposed tire (B) shows the percent change in diameter — and since a speedometer is calibrated to the original tire's revolutions per mile, that same ratio tells you how far off an indicated speed will be once the new tire is mounted.
Worked example
With the defaults — tire A at 225/45R17 and tire B at 235/40R18 — tire A's sidewall is 225 × 0.45 = 101.25mm (3.99 in), giving a diameter of 17 + 2 × 3.99 = 24.97 inches. Tire B's sidewall is 235 × 0.40 = 94mm (3.70 in), giving a diameter of 18 + 2 × 3.70 = 25.40 inches — a difference of +1.72%. Because tire B rolls farther per revolution, driving at a true 61.0 mph will show as an indicated 60 mph on the speedometer once it's installed.
Frequently asked questions
Why does a tire size change affect my speedometer?
Your speedometer doesn't measure speed directly — it counts wheel revolutions and converts that count to a speed using the original tire's overall diameter. A larger tire covers more ground per revolution, so the wheel spins slower than the speedometer expects at any given real speed, and the speedometer under-reads. With this tool's defaults (a 225/45R17 swapped for a 235/40R18), diameter A comes out to about 24.97 inches and diameter B about 25.40 inches — a 1.72% increase. Because the new tire travels farther per rotation, driving at a genuine 61.0 mph will show as an indicated 60 mph on the dash; put another way, when the speedometer reads 60 mph you're actually moving at about 61.0 mph.
What's considered a "safe" tire size difference?
Many tire professionals suggest keeping the overall diameter change within about 3% to avoid meaningful speedometer error, ABS/traction-control miscalibration, and handling or clearance surprises. That's commonly-cited shop guidance, not a hard engineering rule or a legal limit, and it varies by vehicle. If your result is close to or above that range, get a second opinion from a tire shop before buying.
Does this calculator account for load rating?
No. Load index and speed rating are separate safety specifications — stamped on the tire alongside the size — that this diameter math doesn't touch at all. A tire can be a great diameter match and still be the wrong load or speed rating for your vehicle, so check those numbers separately against your door-jamb sticker or owner's manual.
Where do I find my current tire size?
It's molded directly into the tire sidewall in the format width/aspectRwheel — for example, 225/45R17 means a 225mm-wide tire, a sidewall height equal to 45% of that width, and a 17-inch wheel diameter. That's exactly the format this calculator's three inputs (width, aspect ratio, wheel diameter) are built from.