Tire Size Calculator — Diameter and Speed Error
Overall diameter from a tire code, plus speedometer and odometer error for a new size
Runs in your browser · nothing is uploaded
Updated 2026-10-04The sizes already filled in (205/55R16 and 225/45R17) are examples. Replace them with the size printed on your tire sidewall.
The code on the tire sidewall. Formats like 205/55R16, 205 55 16 or P205/55ZR16 all work.
Leave blank to calculate only the current tire.
The speed your dashboard shows, used for the comparison. Enter a whole number from 1 to 400.
Current diameter 631.9 millimetres, new diameter 634.3 millimetres, difference +0.38%
Current tire dimensions 205/55R16
- Sidewall height
- 112.75 mm
- Overall diameter
- 631.9 mm
- Circumference
- 1,985.2 mm
- Revolutions per km
- 503.7
- Section width
- 205 mm
New tire dimensions 225/45R17
- Sidewall height
- 101.25 mm
- Overall diameter
- 634.3 mm
- Circumference
- 1,992.7 mm
- Revolutions per km
- 501.8
- Section width
- 225 mm
Current vs new tire
The new tire is 2.4 mm (0.38%) larger, so the speedometer reads lower than your true speed.
- Diameter difference
- +2.4 mm (+0.38%)
- Sidewall height difference
- -11.5 mm
- Section width difference
- +20 mm
- Rim diameter difference
- +1"
- True speed when the dash shows 100 km/h
- 100.4 km/h
- Odometer reading for 100 km actually driven
- 99.6 km
- Odometer error
- -0.38%
The math with your numbers
- Sidewall = 205 x 55% = 112.75 mm
- Diameter = 16 x 25.4 + 2 x 112.75 = 631.9 mm
- Circumference = pi x 631.9 = 1,985.2 mm, so 1 km = 503.7 turns
- Diameter difference = 634.3 - 631.9 = +2.4 mm (+0.38%)
- True speed = 100 x 634.3 / 631.9 = 100.4 km/h
These figures come from the size code alone. Real tires differ a little by brand, model, inflation pressure, wear and load. Whether a different size fits your car, and how much diameter change is acceptable, depends on the vehicle, so this tool states no limit. Check your owner manual, the manufacturer, a tire shop or your local inspection rules.
What it is
When you are thinking about a different tire size, the first questions are how much the overall diameter changes and how far the speedometer will drift. This tire size calculator reads the code printed on the sidewall, such as 205/55R16, and works out sidewall height, overall diameter, circumference and revolutions per kilometre. Add a second size and it compares the two, showing the diameter difference, your true speed when the dashboard reads a chosen number, and the odometer error. Example sizes are filled in so you see a result right away, and every calculation runs in your browser.
How to use
- Type your current tire size from the sidewall.
- Type the size you are considering. Leave it blank to see only the current tire.
- Set the dashboard speed to compare, as a whole number. The default is 100 km/h.
- Read the diameter difference, true speed and odometer error.
- Check the math lines at the bottom to see how each number was reached.
How it works
- Sidewall height = section width x aspect ratio / 100, in mm.
- Overall diameter = rim diameter x 25.4 + 2 x sidewall height, in mm. Lengths are handled as whole hundredths of a millimetre, so there is no rounding drift.
- Circumference = pi x diameter, and revolutions per km = 1,000,000 / circumference. Pi is used only for these two figures.
- Diameter difference in percent = (new - current) / current x 100, rounded to two decimals.
- True speed = dashboard speed x new diameter / current diameter, shown to one decimal.
- Odometer: for 100 km actually driven it reads current / new x 100 km. The error is that ratio minus one, and a negative value means it under-reads.
- Accepted ranges are width 105 to 405 mm, aspect 20 to 95 percent and rim 10 to 30 inches.
- No tolerance limit is given, because it varies by vehicle.
Examples
Changing from 205/55R16 to 225/45R17 with the dashboard at 100 km/h.
| Item | 205/55R16 | 225/45R17 |
|---|---|---|
| Sidewall height | 112.75 mm | 101.25 mm |
| Overall diameter | 631.9 mm | 634.3 mm |
| Diameter difference | +2.4 mm (+0.38%) | |
| True speed at 100 km/h shown | about 100.4 km/h | |
| Odometer for 100 km driven | about 99.6 km |
These sizes are an example and are not a statement that they fit any particular car.
Good to know
Two tires with the same code can differ slightly in real diameter by brand and model, and inflation pressure, wear and load all move the number. Treat the result as a baseline from the size code. A larger wheel with a lower profile is a common way to keep the diameter close, but a wider tire can bring clearance and approved-rim-width issues that this calculator does not check. Before buying, confirm the size with your owner manual and a tire shop.
FAQ
What does 205/55R16 mean?
205 is the section width in millimetres, 55 is the aspect ratio in percent, R means radial construction and 16 is the wheel (rim) diameter in inches. The aspect ratio is sidewall height as a share of width, so the sidewall here is 205 x 55% = 112.75 mm.
How is the overall diameter calculated?
Convert the rim from inches to millimetres by multiplying by 25.4, then add two sidewalls. For 205/55R16 that is 16 x 25.4 = 406.4 mm plus 2 x 112.75 = 225.5 mm, which gives 631.9 mm.
Why does a different tire change the speedometer?
The speedometer and odometer count wheel turns. A taller tire covers more ground per turn, so your true speed is higher than the dashboard shows, and a smaller tire does the opposite. The calculator multiplies the displayed speed by the new-to-old diameter ratio.
How much diameter change is acceptable?
That depends on the vehicle and on local rules, so this tool gives no limit. Check your owner manual, the manufacturer, a tire shop or your inspection rules. The calculator only reports the difference in millimetres and percent.
Which formats can I type?
205/55R16, 205 55 16, 205-55-16, P205/55ZR16 and LT235/75R15 all work, in any letter case. A trailing load index and speed symbol such as 91V is ignored.
