Tire Size Calculator
Turn a tire size code like 225/65R17 into theoretical dimensions, and compare two sizes to see the difference in diameter and how far your speedometer will read off.
Enter your current and new tire sizes to compare width, sidewall, overall diameter and revolutions per mile, and see how far the speedometer is off. For example, 235/60R18 in place of 225/65R17 is 2.06% larger in diameter, so if the speedometer was set for the old size, an indicated 60 is really 61.2.
This calculator gives theoretical dimensions from the size code only and is not a substitute for a professional fitting. It does not check fitment, load capacity, speed rating or clearance, and real tires differ slightly from these figures, so confirm any size change with your vehicle's maker or a qualified tire fitter.
Diameter difference, new against current
+2.06%
If the speedometer is set for the current size, at 60 you are really going 61.2.
| 225/65R17 | 235/60R18 | |
|---|---|---|
| Section width | 225.0 mm (8.86 in) | 235.0 mm (9.25 in) |
| Sidewall height | 146.3 mm (5.76 in) | 141.0 mm (5.55 in) |
| Overall diameter | 724.3 mm (28.52 in) | 739.2 mm (29.10 in) |
| Circumference | 2,275.5 mm (89.58 in) | 2,322.3 mm (91.43 in) |
| Revolutions per mile | 707.3 | 693.0 |
Theoretical sizes from the code. A real tire's measured and loaded diameters differ a little; check the maker's data and your vehicle's approved sizes.
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About the Tire Size Calculator
A metric tire size packs several measurements into one code. In 215/65R15, 215 is the section width in millimeters, 65 is the aspect ratio, meaning the sidewall is 65% as tall as the section width, R means radial construction, and 15 is the wheel diameter in inches. So the sidewall is 215 x 0.65 = 139.75 mm, and the overall diameter is two sidewalls plus the wheel: 2 x 139.75 + 15 x 25.4 = 660.5 mm, or 26.00 in. Off-road flotation sizes such as 35x12.50R20 skip the math and state the diameter and width in inches. Changing size changes how far the car travels per wheel turn. A taller tire covers more ground each turn, so a speedometer set for the old size reads low, and so does the odometer; a shorter one makes them read high. These are the theoretical figures the code describes; a tire's measured diameter varies by brand and model, and it flattens slightly under load.
Frequently asked questions
What do the numbers on a tire size mean?+
In 225/65R17, 225 is the width in millimeters, 65 is the sidewall height as a percentage of that width, R stands for radial, and 17 is the wheel diameter in inches. Letters in front, such as P for passenger or LT for light truck, give the tire's intended use.
How do I work out a tire's diameter?+
Multiply the width by the aspect ratio and divide by 100 to get the sidewall height in millimeters. Double it, then add the wheel size converted to millimeters at 25.4 mm per inch. For 215/65R15 that gives 660.5 mm, about 26 inches.
Will bigger tires change my speedometer?+
Yes. The speedometer counts wheel turns, so a taller tire makes it read slower than you are going. With 235/60R18 in place of 225/65R17, an indicated 60 is really 61.2. If the speedometer was set for the current size, the calculator shows the true speed for any reading you enter.
Can I fit any size that the calculator compares?+
No. The numbers here only describe size. Whether a tire fits also depends on wheel width, offset, brake and suspension clearance, load index and speed rating. Check the tire placard on the driver's door jamb, the vehicle maker's approved sizes and a tire professional before you change size.
Size code and diameter formula: Tire code (Wikipedia), which gives the theoretical diameter as twice the sidewall height plus the rim diameter x 25.4 mm. 49 CFR 571.139 (FMVSS No. 139), S3 Definitions, defines section width and outer diameter as measured on an inflated tire; the figures here are the theoretical values those measurements approximate.
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