Calculate tire dimensions and compare two tire sizes to check speedometer accuracy and clearance compatibility.
Enter tire size in format: 205/55R16 (Width/Aspect Ratio R Rim)
| Tire 1: 205/55R16 | |
|---|---|
| Width | 205 mm |
| Aspect Ratio | 55% |
| Rim Diameter | 16" |
| Sidewall Height | 4.439" |
| Overall Diameter | 24.878" |
| Circumference | 78.156" |
| Revolutions/Mile | 810.7 |
| Tire 2: 225/45R17 | |
|---|---|
| Width | 225 mm |
| Aspect Ratio | 45% |
| Rim Diameter | 17" |
| Sidewall Height | 3.986" |
| Overall Diameter | 24.972" |
| Circumference | 78.453" |
| Revolutions/Mile | 807.6 |
| P | Vehicle type (P=Passenger, LT=Light Truck) |
| 205 | Section width in mm |
| 55 | Aspect ratio (sidewall % of width) |
| R | Construction (R=Radial) |
| 16 | Rim diameter in inches |
| Size | Common Vehicles |
|---|---|
| 205/55R16 | Honda Civic, Toyota Corolla |
| 215/60R16 | Honda Accord, Toyota Camry |
| 225/65R17 | Honda CR-V, RAV4 |
| 235/55R18 | Ford F-150, Chevy Equinox |
| 245/45R18 | BMW 3 Series, Audi A4 |
| 255/50R19 | Range Rover Sport |
| 265/70R17 | F-150, Silverado |
| 275/55R20 | RAM 1500, Tundra |
Moving from 16-inch to 17-inch wheels does not, by itself, throw off your speedometer. What matters is the overall height of the tire, and a lower sidewall on a bigger rim can cancel the extra rim size almost exactly: a 205/55R16 stands 24.88 inches tall, a 225/45R17 stands 24.97, a difference of 0.38%. This calculator reads two tire sizes, works out each one's true diameter, and shows what the speedometer will claim afterward.
The steps: sidewall (in) = width (mm) × aspect ÷ 100 ÷ 25.4. Diameter = rim + 2 × sidewall. Circumference = π × diameter. Revolutions per mile = 63,360 ÷ circumference. True speed = indicated speed × (new circumference ÷ old circumference).
Example: 205/55R16 has a sidewall of 4.439 in, a diameter of 24.878 in, a circumference of 78.156 in and 810.7 revolutions per mile. The 225/45R17 has a 3.986 in sidewall, a 24.972 in diameter, 78.453 in around and 807.6 revolutions per mile. The ratio 78.453 ÷ 78.156 is 1.0038, so a speedometer reading of 60 mph means 60.2 mph (60.23). Diameter is up 0.38% and revolutions per mile are down 0.38%. On the odometer, 12,000 indicated miles would really be about 12,046.
Compare that with a 225/50R17, which keeps a taller sidewall: 25.858 in, up 3.94%, and 62.4 mph on the road when the dial says 60. The tile turns red when the difference is more than 2 mph.
What it does not account for: the numbers are nominal geometry, not measured tires. Brands differ, tread wear shrinks the diameter, and inflation and load change it slightly. The page does not check rim width, load index, speed rating, offset or fender clearance, and it does not model gearing. Sizes shown to three decimals, speeds to one decimal, and percentages to two.
A widely quoted rule of thumb is to stay within about 3% of the original diameter. That is only a guideline: check your vehicle's manual or a tire shop, since traction control, ABS and gearing can be sensitive to it.
A taller tire covers more ground per turn, so the speedometer reads low and your real speed is higher. A shorter tire does the opposite. Many speedometers already read slightly high from the factory, so the two effects partly offset.
Yes, if you keep the aspect ratio. Going from 205/55R16 to 225/55R16 raises the sidewall to 4.872 in and the diameter to 25.74 in, an increase of 3.5%. Most people who go wider drop the aspect ratio to compensate.
The field accepts one optional leading letter, then width, aspect ratio, R or a slash, and rim size. Sizes like LT265/70R17, 225/45ZR17 or 35x12.50R17 are not read, so enter the equivalent without the prefix.
Yes. It counts wheel turns just like the speedometer. With a taller tire the odometer reads low, here by about 0.38%, so 12,000 indicated miles are really about 12,046. That matters for mileage-limited leases and warranties.