How to use the tire size calculator
- Find your current size on the tire sidewall or the sticker inside the driver’s door — for example 225/45R17.
- Enter the width, aspect ratio and rim size of the current tire and the tire you are considering.
- Enter a speedometer reading to see your real speed with the new tires.
The formulas
Sidewall height = Width × Aspect ratio ÷ 100 Diameter = Rim × 25.4 + 2 × Sidewall (mm) Circumference = π × Diameter Revs per mile = 1,609,344 mm ÷ Circumference Difference % = (New diameter ÷ Old diameter − 1) × 100 Actual speed = Speedometer reading × New diameter ÷ Old diameter
Worked example
Changing from 225/45R17 to 225/50R17:
- Old sidewall = 225 × 0.45 = 101.25 mm → diameter = 431.8 + 202.5 = 634.3 mm (24.97 in)
- New sidewall = 225 × 0.50 = 112.5 mm → diameter = 431.8 + 225 = 656.8 mm (25.86 in)
- Difference = 656.8 ÷ 634.3 − 1 = +3.55%
- At an indicated 60 mph you are really doing 62.1 mph, and the car rides 0.44 in (11 mm) higher
Common sizes and their diameters
| Size | Diameter | Typical vehicle |
|---|---|---|
| 195/65R15 | 25.0 in (634 mm) | Compact cars |
| 205/55R16 | 24.9 in (632 mm) | Compact and mid-size cars |
| 225/45R17 | 25.0 in (634 mm) | Sport compacts, sedans |
| 235/65R17 | 29.0 in (737 mm) | Crossovers and SUVs |
| 265/70R17 | 31.6 in (803 mm) | Pickups and off-road SUVs |
| 275/60R20 | 33.0 in (838 mm) | Full-size pickups |
| 285/70R17 | 32.7 in (831 mm) | Lifted trucks, Jeeps |
What changes when you change tire size
- Speedometer and odometer: taller tires read slow and under-count miles; shorter tires read fast. A dealer or tuner can recalibrate many vehicles.
- Gearing and fuel economy: larger, heavier tires make acceleration slower and usually raise fuel use.
- Clearance: wider or taller tires can rub on suspension parts, fenders or liners at full steering lock or under compression.
- Load and speed rating: the new tire must have at least the load index and speed rating listed on your door placard.
- All-wheel drive: tires on the same vehicle must match closely — mixing diameters can damage AWD and 4×4 systems.
Flotation and LT sizes
Off-road and truck tires are often sold in flotation sizes such as 33×12.50R15, where 33 is the diameter in inches, 12.50 the width in inches and 15 the rim. These don’t need converting — the first number is already the overall diameter. LT-metric sizes such as LT265/75R16 use the same formula as passenger sizes in this calculator: 16 × 25.4 + 2 × 265 × 0.75 = 803.9 mm, or 31.6 in.
Checking the result on the road
After fitting new tires, compare your speedometer with a GPS speed app on a straight, level road at a steady speed. Most speedometers already read slightly high from the factory, so a small increase in tire diameter can actually make the reading more accurate. Also check the spare tire: if it no longer matches the new size, it should only be used briefly and at low speed.