Tire Size Calculator
How tall is my tire, and what happens if I change sizes?
A P265/70R17 tire is about 31.6 inches tall; switching to 275/65R18 makes it about 1.5% taller, so 60 mph on the speedometer is really about 60.9 mph.
Overall diameter = rim × 25.4 mm + 2 × width × aspect ÷ 100 ·
Current tire overall diameter
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- Width · aspect · wheel
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- Sidewall height
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- Circumference
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- Revolutions per mile
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Diameter difference with the new tire
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- New tire diameter
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- New sidewall height
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- Difference
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- Revolutions per mile
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- Odometer reads 10,000 mi, actual
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- Actual speed
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±3% is a common rule of thumb, not a legal limit. Check your vehicle maker’s approved sizes, load index and speed rating too.
How to read a tire size
| P265/70R17 115T | Meaning |
|---|---|
| P | Passenger tire (LT = light truck, no letter = Euro-metric) |
| 265 | Section width in millimeters |
| 70 | Aspect ratio — sidewall height as a % of width |
| R | Radial construction (ZR = radial rated for high speed) |
| 17 | Wheel (rim) diameter in inches |
| 115 · T | Load index · speed rating (not used for size) |
Flotation sizes like 33x12.50R15 read as overall diameter × width × wheel size, all in inches. Real tires can differ from the nominal size by a few millimeters depending on brand, inflation and wear.
Something off with the result?
How it's calculated
Overall diameter = rim × 25.4 mm + 2 × width × aspect ÷ 100
- Metric sizes (P-metric, LT-metric, Euro-metric): section width (mm) / aspect ratio (%) construction wheel diameter (in)
- Sidewall = width × aspect ratio ÷ 100, overall diameter = wheel diameter × 25.4 mm + 2 × sidewall, circumference = π × diameter
- Flotation sizes (33x12.50R15): overall diameter and width as marked, sidewall = (diameter − wheel) ÷ 2
- Revolutions per mile = 1,609,344 mm ÷ circumference
- Speedometer: assumes it was calibrated to the original tire, so actual speed = reading × (new diameter ÷ original diameter). The odometer is off by the same ratio
- ±3% is a widely used industry rule of thumb, not a regulation. Allowed ranges vary by vehicle
- 1 inch = 25.4 mm. Calculations use nominal dimensions
Frequently asked questions
What do the numbers on a tire mean?
On P265/70R17 115T, “P” means passenger tire (LT is light truck), 265 is the section width in millimeters, 70 is the aspect ratio (sidewall height is 70% of the width), R means radial construction and 17 is the wheel diameter in inches. 115T is the load index and speed rating, which don’t affect the size.
How do you calculate tire diameter?
Sidewall height = width × aspect ratio ÷ 100, and overall diameter = wheel diameter × 25.4 mm + 2 × sidewall. For P265/70R17 that is 185.5 mm of sidewall and 802.8 mm overall, or about 31.6 inches.
Will bigger tires throw off my speedometer?
Yes. The speedometer counts wheel turns based on the original tire. Going from P265/70R17 to 275/65R18 makes the tire about 1.5% taller, so at 60 mph on the speedometer you’re really doing about 60.9 mph, and the odometer under-counts miles by the same amount.
How much bigger can I go on tire size?
A common rule of thumb is to stay within 3% of the original overall diameter. It isn’t a legal limit, and clearance, load index, ABS and stability control can all set tighter limits. Check the tire placard on the driver’s door jamb and your owner’s manual first.
What is a 33 inch tire in metric?
Flotation sizes such as 33x12.50R15 give the overall diameter (33 in), the width (12.5 in) and the wheel size (15 in) directly. 33 inches is 838.2 mm. Metric sizes close to 33 inches include 285/75R16 and 275/70R18. You can type either format into this calculator.