Current tire (the three numbers on the sidewall, e.g. 225/45R17)
New tire
A tire size such as 225/45R17 has three numbers. 225 is the tread width in millimetres. 45 is the aspect ratio: the sidewall height as a percentage of the width (here 45% of 225 mm = 101 mm). 17 is the wheel (rim) diameter in inches. The overall diameter is rim diameter in mm plus two sidewalls: 17 x 25.4 + 2 x 101.25 = 634 mm (25.0 in). Your speedometer and odometer assume the original diameter, so a taller tire makes the car go faster than the dial says and a shorter one makes it go slower. Real speed = shown speed x new diameter / old diameter. Most tire experts suggest staying within about 3% of the original diameter so the speedometer, gearing, ABS and traction control keep working as designed.
| Size | Sidewall | Overall diameter | Circumference | Revs per mile |
|---|---|---|---|---|
| 195/65R15 | 127 mm | 635 mm (25.0 in) | 1993 mm | 807 |
| 205/55R16 | 113 mm | 632 mm (24.9 in) | 1985 mm | 811 |
| 215/60R16 | 129 mm | 664 mm (26.2 in) | 2087 mm | 771 |
| 225/45R17 | 101 mm | 634 mm (25.0 in) | 1993 mm | 808 |
| 225/65R17 | 146 mm | 724 mm (28.5 in) | 2275 mm | 707 |
| 235/55R18 | 129 mm | 716 mm (28.2 in) | 2248 mm | 716 |
| 245/40R18 | 98 mm | 653 mm (25.7 in) | 2052 mm | 784 |
| 255/55R18 | 140 mm | 738 mm (29.0 in) | 2318 mm | 694 |
| 265/70R17 | 186 mm | 803 mm (31.6 in) | 2522 mm | 638 |
| 275/55R20 | 151 mm | 811 mm (31.9 in) | 2546 mm | 632 |
| 285/75R16 | 214 mm | 834 mm (32.8 in) | 2620 mm | 614 |
| 265/65R18 | 172 mm | 802 mm (31.6 in) | 2519 mm | 639 |
In 225/45R17, 225 is the width in mm, 45 is the sidewall height as a percent of the width, R means radial construction and 17 is the wheel diameter in inches.
Keep the overall diameter within about 3% of the original size. A bigger difference makes the speedometer and odometer noticeably wrong and can interfere with ABS, traction control and gearing.
A larger overall diameter covers more ground per wheel turn, so the speedometer reads lower than your real speed. With 3% taller tires, a 60 mph reading means about 61.8 mph.
Plus-sizing means fitting a larger wheel with a lower-profile tire so the overall diameter stays about the same, for example 205/55R16 to 225/45R17. This calculator shows how close the new combination gets.
Check your vehicle’s load index, speed rating and clearance, and the maker’s approved sizes, before changing tire sizes.
See also: speed, distance, time calculator · pace calculator · fuel cost calculator · unit converters · all tools