Tire size calculator

First find your current tire size: it is molded into the sidewall (for example 225/45R17) and also printed on the tire placard inside the driver's door jamb and in the owner's manual.

1. Tire 1 (current): enter the section width in mm (225), the aspect ratio in % (45) and the wheel diameter in inches (17). 2. Tire 2 (new): enter the size you are considering, for example 245/40R18. 3. Read the results: the overall diameter of each tire in millimeters, the percent difference between them, and the actual road speed when your speedometer reads 100 with the new size installed.

If the difference stays within ±3%, the swap is generally considered equivalent for everyday driving. Also check the load index and speed rating (for example 91V): the new tire should meet or exceed the original values, which this calculator does not evaluate because it depends on the specific tire model. Truck owners going up in size should additionally check fender and suspension clearance at full steering lock.

The overall diameter of a tire is the wheel diameter (converted to millimeters) plus twice the sidewall height:

`` diameter (mm) = rim × 25.4 + 2 × (width × aspect / 100) difference (%) = (d2 − d1) / d1 × 100 actual speed at an indicated 100 = 100 × d2 / d1 ``

Worked example with a common American plus-size, a Honda CR-V moving from 225/65R17 to 235/60R18:

• Tire 1: 17 × 25.4 = 431.8 mm of wheel; sidewall = 225 × 65/100 = 146.25 mm; diameter = 431.8 + 2 × 146.25 = 724.3 mm (28.5 in).
• Tire 2: 18 × 25.4 = 457.2 mm; sidewall = 235 × 60/100 = 141 mm; diameter = 457.2 + 2 × 141 = 739.2 mm (29.1 in).
• Difference: (739.2 − 724.3) / 724.3 × 100 = +2.06%.
• Speedometer: at an indicated 100 mph the true speed is 100 × 739.2 / 724.3 = 102.1 mph — the speedometer under-reads because the taller tire covers more ground per revolution.

At +2.06% this swap stays inside the 3% rule of thumb, but it is close enough that recalibrating the odometer reading matters if you track fuel economy.

The table shows swaps that come up constantly in American forums and tire shops, with each overall diameter, the percent difference and the true speed at an indicated 100 mph:

Note the F-150 example: going from 265 to 285 width on the same 17-inch wheel pushes the diameter up 3.49%, just past the common 3% guideline. The truck will read about 3.5 mph slow at highway speed and the odometer will under-count miles — something to remember at trade-in time and when calculating real-world MPG.

The recommended tolerance is ±3% in overall diameter versus the original size. Beyond that, problems start stacking up:

• Speedometer and odometer: with a taller tire the speedometer reads lower than your true speed (hello, speeding tickets) and the odometer logs fewer miles than you actually drive. A shorter tire does the opposite — and quietly inflates the mileage on a leased vehicle.
• Fuel economy math: the trip computer divides fuel used by odometer miles, so taller tires make your MPG look worse than it really is.
• Electronics: ABS, stability control and TPMS are calibrated for the factory diameter; large deviations can affect how they intervene.
• Clearance: wider or taller tires can rub the fender liner or suspension at full lock, especially on lifted trucks.

In the U.S. there is no federal limit on tire size, but several states inspect for tires protruding past the fenders or modified suspensions, and an insurer can raise questions after an accident if the vehicle was running a size far outside the manufacturer's specification. Most OEMs and tire retailers stick to the 3% guideline. Typical input mistakes with this calculator: mixing up width (mm) with wheel diameter (inches), entering the aspect ratio as millimeters instead of a percentage, and forgetting that an LT-metric tire of the same nominal size can measure slightly different from its P-metric counterpart.