Quick Answer
- A metric size converts to inches with diameter = rim + 2 x width x aspect / 100 / 25.4. A 225/45R17 is 25.0 in tall and 8.9 in wide, written 25.0x8.9R17 in flotation form (calculated).
- Compared with a 235/45R17 (25.3 in) it is 0.35 in shorter: 1.4% on the speedometer and 808 against 796 revs per mile.
- Inch sizes are nominal: for 33x12.50R15 the metric match in the chart below is 315/70R15 (calculated), and makers build the same inch size a little differently.
Tire 1
P-Metric presets:
Tire 2
P-Metric presets:
Within 3% (Continental)
+1.42%Diameter difference: +0.35" (+1.4%): within the 3% rolling circumference limit in Continental's guidance
225/45R17 Ø
24.97"
634.3mm
235/45R17 Ø
25.33"
643.3mm
Ø Difference
+0.35"
9.0mm
Speedo Error
+1.42%
Speedo reads LOW (taller tire)
Continental advises keeping the rolling circumference within 3% of the original.
| Specification | Tire 1 | Tire 2 | Difference |
|---|---|---|---|
| Section Width (mm) | 225 | 235 | +10 (+4.4%) |
| Aspect Ratio (%) | 45.0 | 45.0 | No change |
| Rim Diameter (in) | 17 | 17 | No change |
| Sidewall Height (mm) | 101.3 | 105.8 | +4.5 (+4.4%) |
| Sidewall Height (in) | 3.986 | 4.163 | +0.177 (+4.4%) |
| Overall Diameter (mm) | 634.3 | 643.3 | +9.0 (+1.4%) |
| Overall Diameter (in) | 24.97 | 25.33 | +0.35 (+1.4%) |
| Circumference (in) | 78.45 | 79.57 | +1.11 (+1.4%) |
| Revs per Mile | 808 | 796 | -11 (-1.4%) |
| Revs per km | 502 | 495 | -7 (-1.4%) |
Difference = Tire 2 − Tire 1 with % change relative to Tire 1. Values are geometric estimates based on nominal sizes.
Get a plain-English explanation of your results and what they mean for your vehicle.
Metric to Flotation Tire Size Chart
Calculated from the size code. The flotation form is the overall diameter and width in inches.
| Metric | Flotation | Diameter in | Width in | Rim in | Revs per mile |
|---|---|---|---|---|---|
| 225/45R17 | 25.0x8.9R17 | 25.0 | 8.9 | 17 | 808 |
| 245/40R18 | 25.7x9.6R18 | 25.7 | 9.6 | 18 | 784 |
| 265/70R17 | 31.6x10.4R17 | 31.6 | 10.4 | 17 | 638 |
| 275/65R18 | 32.1x10.8R18 | 32.1 | 10.8 | 18 | 629 |
| 275/60R20 | 33.0x10.8R20 | 33.0 | 10.8 | 20 | 611 |
| 285/70R17 | 32.7x11.2R17 | 32.7 | 11.2 | 17 | 617 |
| 295/70R18 | 34.3x11.6R18 | 34.3 | 11.6 | 18 | 589 |
| 305/55R20 | 33.2x12.0R20 | 33.2 | 12.0 | 20 | 607 |
| 315/70R17 | 34.4x12.4R17 | 34.4 | 12.4 | 17 | 587 |
Flotation to Metric: 33x12.50R15 and Other Common Sizes
The closest metric size for each inch size, with the diameter of the metric size and its difference from the nominal inch height. Calculated; confirm exact sizes with the tire maker.
| Flotation | Metric | Metric diameter | Difference | Width |
|---|---|---|---|---|
| 31x10.50R15 | 265/75R15 | 30.6" | -0.4" | 10.5" |
| 33x12.50R15 | 315/70R15 | 32.4" | -0.6" | 12.5" |
| 33x12.50R17 | 315/65R17 | 33.1" | +0.1" | 12.5" |
| 35x12.50R15 | 315/80R15 | 34.8" | -0.2" | 12.5" |
| 35x12.50R17 | 315/70R17 | 34.4" | -0.6" | 12.5" |
| 35x12.50R20 | 295/65R20 | 35.1" | +0.1" | 12.5" |
| 37x12.50R17 | 315/80R17 | 36.8" | -0.2" | 12.5" |
The full flotation chart is on the metric tire calculator. For a pickup application see the truck tire size calculator.
Tire Size Comparison Examples
Calculated differences between common swaps. For any pair use the converter above, or the tire size comparison calculator.
| From | To | Diameter change | Speedometer error | Width change | True speed at 60 mph |
|---|---|---|---|---|---|
| 225/45R17 | 235/45R17 | +0.35 in | +1.4% | +0.39 in | 60.9 mph |
| 265/70R17 | 285/70R17 | +1.10 in | +3.5% | +0.79 in | 62.1 mph |
| 275/65R18 | 295/65R18 | +1.02 in | +3.2% | +0.79 in | 61.9 mph |
| 265/70R17 | 315/70R17 | +2.76 in | +8.7% | +1.97 in | 65.2 mph |
| 255/55R18 | 265/50R19 | +0.39 in | +1.3% | +0.39 in | 60.8 mph |
Conversion Formulas and Sources
Metric to inches
Sidewall = width x aspect / 100. Diameter = rim + 2 x sidewall / 25.4. 225/45R17: sidewall 101.25 mm, diameter 24.97 in, width 8.86 in.
Inches to metric
Width in mm = inches x 25.4, rounded to 5. Sidewall = (diameter - rim) / 2, and aspect = sidewall in mm / width x 100, rounded to 5. 33x12.50R15 gives 315/70R15 here.
Speedometer error
The percentage difference in overall diameter. Actual speed = indicated speed x new diameter / old diameter.
Revs per mile
Revs per mile = 63,360 / (pi x diameter in inches).
| Source | Statement (paraphrased) |
|---|---|
| Continental Tire, plus-sizing guidance | When changing tire size keep the overall rolling circumference within 3% of the original, and use only approved size and rim width combinations. |
| USTMA, Care and Service of Passenger and Light Truck Tires | Acceptable load capacity or overall diameter does not imply acceptable vehicle clearance. Check clearance at all four wheel positions through full suspension travel and steering lock. |
| This site's conversion method | Metric to inches uses the geometry of the size code. Inches to metric keeps the flotation width, rounded to the nearest 5 mm, and picks the aspect ratio that matches the nominal height (src/lib/flotationMetricData.ts). |
How to Use the Tire Size Converter
- 1Choose metric or flotation for your current tire and enter its size, for example 265/70R17 or 33x12.50R15.
- 2Choose the format for the second tire and enter the size you are considering.
- 3Read the Stats tab for diameter, width, sidewall, revs per mile and speedometer error, with the equivalent in the other format.
- 4Use the Visual, Speedometer and Graphs tabs to see the difference to scale.
- 5Check clearance and the approved rim width before buying.