Calculator Tools
Tire Size Calculator
Parse any P-metric tire size, get overall diameter, sidewall, circumference, and revs per mile, then compare two sizes with speedometer error.
Accepted formats: 215/65R17, P215/65R17, 225/45ZR18, LT265/75R16. Letters and spaces are normalized automatically.
Enable to see diameter delta, sidewall change, ground-clearance change, and speedometer error if the new tire replaces the old.
Tire A dimensions
215/65R17
- Section width215 mm (8.46 in)
- Aspect ratio65%
- Sidewall height139.75 mm (5.5 in)
- Wheel diameter17 in
- Overall diameter28 in (711.3 mm)
- Circumference87.98 in (2234.61 mm)
- Revs / mile720.2
- Revs / kilometer447.5
Tire B dimensions
235/65R17
- Section width235 mm (9.25 in)
- Aspect ratio65%
- Sidewall height152.75 mm (6.01 in)
- Wheel diameter17 in
- Overall diameter29.03 in (737.3 mm)
- Circumference91.19 in (2316.3 mm)
- Revs / mile694.8
- Revs / kilometer431.7
Comparison
215/65R17 vs 235/65R17
Deltas show how Tire B differs from Tire A. Positive means Tire B is larger.
- Overall diameter change+1.02 in (+26 mm)
- Percent change+3.66%
- Section width+20 mm
- Sidewall height+13 mm
- Ground clearance change+0.51 in (+13 mm)
- Speed ratio (actual / indicated)1.0366
Speedometer error
If the speedometer was calibrated for 215/65R17 and you install 235/65R17, an indicated speed of N will read as the actual speed below.
| Indicated | Actual | Difference |
|---|---|---|
| 30 mph / 30 km/h | 31.1 mph / 31.1 km/h | +1.1 |
| 50 mph / 50 km/h | 51.8 mph / 51.8 km/h | +1.8 |
| 60 mph / 60 km/h | 62.2 mph / 62.2 km/h | +2.2 |
| 70 mph / 70 km/h | 72.6 mph / 72.6 km/h | +2.6 |
| 80 mph / 80 km/h | 82.9 mph / 82.9 km/h | +2.9 |
Most manufacturers recommend keeping the overall diameter change within 3 percent of the original size to avoid clearance issues and large speedometer errors. The result above shows +3.66%.
What the numbers in a tire size mean
Section width
215
Tread width in millimeters, sidewall to sidewall.
Aspect ratio
65
Sidewall height as a percent of section width.
Wheel diameter
R17
Rim diameter in inches. R is for radial construction.
Optional leading letters (P, LT, ST, T) describe vehicle class and are ignored for the math. Optional trailing load index and speed rating (for example 95H) are also ignored.
How to use
- Type the original tire size into the Tire A field. The format is width/aspect, then R, then wheel diameter, for example 215/65R17. Letter prefixes such as P or LT are accepted and ignored.
- Read Tire A dimensions for section width, aspect ratio, sidewall, overall diameter, circumference, and revs per mile and per kilometer.
- Keep the Compare a second tire size option enabled, then enter Tire B. Use the preset chips to load common sizes quickly.
- Review the Comparison card for diameter delta, percent change, ground-clearance change, and the speedometer error table at common road speeds.
- Click Copy full summary to put both sets of dimensions, the deltas, and the speedometer table on your clipboard.
About this tool
Tire Size Calculator parses any standard P-metric or metric tire size (for example 215/65R17, P225/45ZR18, LT265/75R16) and computes every physical dimension that follows from the three numbers: section width in millimeters and inches, aspect ratio as a percent, sidewall height in millimeters and inches, wheel rim diameter, overall tire diameter, circumference, revolutions per mile, and revolutions per kilometer. Enter a second size and the tool compares the two side by side, showing the overall diameter delta in inches and millimeters, the percent change, the section width and sidewall differences, the ground-clearance change (half of the diameter delta), and the speedometer error if the new tire replaces the old. A speed table reports the actual speed for indicated speeds from 30 to 80, in both mph and km/h, so plus sizing decisions are easy to validate. The tool also flags whether the percent change stays within the 3 percent guideline most manufacturers cite as a safe range for clearance and electronics calibration. All math is exact (1 inch = 25.4 mm, 1 mile = 63360 inches, 1 km = 1,000,000 mm). Optional leading letters such as P, LT, ST, and T are recognized and ignored, as are trailing load index and speed rating codes, so most real tire sidewall markings parse on the first try. Everything runs in your browser; nothing about your vehicle or shopping list is uploaded and no signup is required. Useful for plus sizing, off-road tire upgrades, classic car restoration, fleet management, speedometer calibration, comparing rotating mass across spare wheels, planning winter and summer setups, and quickly answering whether two tires are close enough in diameter to mix on the same axle.
Free to use. Works in your browser. No signup, no login.
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