Tire size calculator and comparison

How to compare two tire sizes

1. Type the size printed on your current tires, for example 245/75R17 or P225/65R17. It is on the sidewall and on the tire placard inside the driver's door jamb.
2. Type the new size. Metric sizes (285/70R17), LT sizes (LT285/75R16) and flotation sizes in inches (33x12.50R17) all work, or you can pick width, aspect ratio and rim one at a time.
3. The drawing overlays both tires in side and front view, so the extra height and width show at a glance.
4. Read the differences in inches, millimeters and percent, and what the speedometer shows at 60 mph.
5. Copy the link to share the exact comparison with a friend, a forum or your tire shop.

Everything is calculated on your device. After the first visit the calculator also works offline, which helps inside a tire shop with no signal.

How the numbers are calculated

A metric size gives the section width in millimeters, the sidewall height as a percentage of that width (the aspect ratio) and the rim diameter in inches. Everything else follows from three formulas:

• Sidewall height = width × aspect ratio ÷ 100.
• Overall diameter = rim diameter × 25.4 + 2 × sidewall height, in millimeters.
• Circumference = π × overall diameter. Revolutions per mile = 1,609,344 ÷ circumference in millimeters.
• True speed = indicated speed × new diameter ÷ original diameter.

The 3% rule: what it is and what it is not

Tire shops and wheel makers usually keep a replacement size within about 3% of the original overall diameter. Inside that window the speedometer, the odometer, the transmission shift points and the wheel speed sensors used by ABS and stability control (ESC) stay close to how the vehicle was calibrated.

In the United States we found no federal rule that limits tire diameter: the 3% figure is industry practice, not law. Some states regulate lift height or tires that stick out past the fenders, so check your state's rules before a big change. Brazil is different: its national traffic rules set a ±3% limit, which is why the Portuguese version of this calculator shows it as a legal reference.

Staying within 3% does not make a size right for your vehicle. Keep the load index and speed rating equal to or higher than the original, make sure the tire clears the fender, brakes and suspension at full steering lock, and confirm with the owner's manual and a tire professional.

Plus sizing: bigger wheels, same overall height

To fit a larger wheel without changing the overall diameter, the sidewall has to get shorter. The usual plus-one pattern is one inch more rim, about 10 to 20 mm more width and 5 to 10 points less aspect ratio. 205/55R16 to 225/45R17 follows it exactly and changes the diameter by only 0.38%. Shorter sidewalls sharpen steering but ride firmer and are easier to damage on potholes.

Example: 33-inch tires on a Wrangler or Tacoma

Many Jeep Wrangler trims come on 245/75R17 tires, 31.47 in (799.3 mm) tall. A 33x12.50R17 is 33.0 in (838.2 mm): 1.53 in (38.9 mm) taller, or +4.87%. At an indicated 60 mph the Jeep is really doing about 62.9 mph, and it sits about 0.77 in (19 mm) higher on the same suspension. That is well past 3%, so the swap usually comes with a lift or leveling kit, a speedometer recalibration and, on smaller engines, new axle gears. Tacoma owners make the same jump from 265/70R16 (30.61 in), which is +7.82% to a true 33: compare your exact sizes above before you buy.

Equivalent sizes

Below the result, the calculator lists sizes with nearly the same overall diameter on other rim sizes. It is a quick way to see your options when plus sizing or when the original size is hard to find. Treat it as a starting point: rim width, offset, load index and clearance still have to match.