Tire size formulas
Sidewall (mm) = width × aspect ÷ 100
Diameter (in) = 2 × sidewall ÷ 25.4 + rim
Circumference = diameter × π · Revs per mile = 63,360 ÷ circumference (in)
Actual speed = shown speed × new diameter ÷ old diameter
Worked example
A 225/65R17 tire has a sidewall of 225 × 0.65 = 146.25 mm (5.76 in), so its diameter is 2 × 5.76 + 17 = 28.5 inches. A 235/65R17 has a 6.01 in sidewall and a 29.0 in diameter, about 1.8% taller. At an indicated 60 mph you'd really be going about 61.1 mph.
Reading a tire sidewall
| Part | Example | Meaning |
|---|
| Type | P / LT | Passenger or light truck |
| Width | 225 | Section width in mm |
| Aspect ratio | 65 | Sidewall height as % of width |
| Construction | R | Radial |
| Rim size | 17 | Wheel diameter in inches |
| Load / speed | 102H | Load index and speed rating |
Calculated sizes are nominal. Actual dimensions vary by brand, tread and wheel width. Converting speeds or distances? Use the unit converter or the speed calculator.
Last reviewed October 2026.
Frequently asked questions
How do I read a tire size like 225/65R17?
225 is the tread width in millimeters. 65 is the aspect ratio: the sidewall height is 65% of the width. R means radial construction, and 17 is the wheel (rim) diameter in inches. A "P" or "LT" in front means passenger or light truck.
How is tire diameter calculated?
Sidewall height = width × aspect ratio ÷ 100, in millimeters. Overall diameter = 2 × sidewall height + rim diameter. The calculator converts millimeters to inches (÷ 25.4) so everything is in the same unit.
How does a new tire size change my speedometer?
The speedometer counts wheel turns. A taller tire travels farther per turn, so you go faster than the speedometer shows. A shorter tire does the opposite. The difference is the same percentage as the change in diameter.
How much size difference is safe?
Many tire shops suggest staying within about 3% of the original overall diameter to avoid problems with speedometer accuracy, ABS, traction control and clearance. Always check your vehicle manual and the tire placard, and ask a tire professional.
What does revolutions per mile mean?
It is how many times the tire turns to travel one mile. Taller tires have fewer revolutions per mile. Manufacturers publish measured figures that can differ slightly from the calculated value because tires flatten under load.