Compare two tire sizes before you buy. Type your current size and the size you are considering, and the calculator shows the overall diameter, width, sidewall height, revolutions per mile, the change in ground clearance and how far your speedometer will be off. It reads metric sizes such as 265/70R17 or P225/65R17 and flotation sizes such as 33×12.50R17.
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Figures are nominal geometry calculated from the size code. A tire’s real measured diameter varies by brand, model and tread depth; check the manufacturer’s spec sheet before you buy, and confirm clearance, load index and speed rating for your vehicle.
How to read the results
- Overall diameter is the height of the tire from the ground to the top of the tread. A taller tire raises the vehicle and changes your gearing and speedometer.
- Section width is the width of the tire at its widest point when mounted on the measuring rim. A wider tire needs more room at the fender, the inner liner and the suspension.
- Sidewall height is the rubber between the wheel and the tread. More sidewall usually means a softer ride and more protection off road; less sidewall sharpens steering.
- Revolutions per mile is how many times the tire turns in one mile. Your speedometer and odometer count these turns, which is why a bigger tire makes them read low.
- Actual speed shows your true speed when the speedometer shows the number you entered, assuming it was calibrated for your current tire.
The formulas behind the calculator
A metric size such as 265/70R17 has three numbers: the section width in millimeters (265), the aspect ratio, which is the sidewall height as a percentage of the width (70), and the wheel diameter in inches (17). The calculator works out:
- Sidewall height = 265 × 70% = 185.5 mm = 7.30 inches
- Overall diameter = wheel diameter + 2 × sidewall = 17 + 2 × 7.30 = 31.61 inches
- Circumference = π × diameter = 99.29 inches
- Revolutions per mile = 63,360 inches in a mile ÷ circumference = 638.1
Flotation sizes such as 33x12.50R17 state the overall diameter (33 inches) and the section width (12.50 inches) directly, so the sidewall is (33 − 17) ÷ 2 = 8 inches.
Worked example: 265/70R17 to 285/70R17
This is a common upgrade on full-size and mid-size trucks. Going from 265/70R17 to 285/70R17 makes the tire 1.10 inches taller (31.61 to 32.71 inches, a 3.49% increase) and 0.79 inches wider. The vehicle sits about 0.55 inches higher, the tire turns 21.5 fewer times per mile, and when the speedometer shows 60 mph you are actually doing about 62.1 mph.
How much bigger can you go?
A widely used guideline is to keep the new overall diameter within about 3% of the original, so the speedometer, the anti-lock brakes and the stability control keep working as designed. Larger changes are common on trucks, but they usually need more clearance (a leveling kit or lift), a speedometer recalibration, and sometimes different axle gearing. Whatever you choose, the new tire must also meet the load index and speed rating your vehicle needs; you can find those on the placard inside the driver's door jamb and in the owner's manual.
Frequently asked questions
Is the calculated diameter the real size of the tire?
No. It is the nominal size from the size code. Real tires of the same size differ by brand, model and tread depth, often by a few tenths of an inch. For an exact figure, check the overall diameter in the manufacturer's specification table for that tire.
What do P and LT in front of a size mean?
P marks a passenger tire and LT a light truck tire built for heavier loads, usually with a stiffer construction. The geometry of P265/70R17 and LT265/70R17 is the same, but their load capacity and recommended inflation are not. Do not replace an LT tire with a P tire on a vehicle that needs the higher load capacity.
Why does my speedometer read slower with bigger tires?
The speedometer counts how fast the wheels or the transmission turn. A taller tire covers more ground per turn, so at the same reading you are moving faster than shown.
Will a 3% bigger tire fit my vehicle?
Not always. Diameter is only part of it: width, wheel offset, the suspension and the steering angle all decide whether a tire rubs. Use our wheel offset calculator if you are also changing wheels, and test-fit before you drive.