How the Tire Size Calculator Works
The numbers on the side of a tire, such as 205/55R16, are a code for its size. They give the width, the height of the sidewall in relation to the width, and the size of the wheel it fits. The Tire Size Calculator turns that code into the overall height of the tire, and lets you compare it with another size. This is useful if you want new wheels, winter tires, or bigger tires, and you want your speedometer to stay right.
How to read a tire size
Take the size 205/55R16:
| Part | Meaning |
|---|---|
| 205 | The width of the tire, in millimeters, from one sidewall to the other. |
| 55 | The aspect ratio. The height of the sidewall is 55% of the width. |
| R | Radial construction, which is the type used on almost all cars. |
| 16 | The diameter of the wheel (the rim), in inches. |
What you enter
- Width, aspect ratio and rim size of your tire.
- A second tire size (optional), to compare with the first.
- A speedometer reading (optional), to see what the real speed is with the new tires.
How the results are worked out
sidewall height = width x aspect ratio / 100 overall diameter = rim diameter + 2 x sidewall height circumference = pi x overall diameter turns per mile = 63,360 inches / circumference
The width and the sidewall are in millimeters, and the rim is in inches, so the calculator changes them to the same unit first. One inch is 25.4 millimeters. The overall diameter is the rim plus two sidewalls, one above and one below the wheel.
A worked example
The tire is 205/55R16.
| Step | Result |
|---|---|
| Sidewall (205 x 55 / 100) | 112.75 mm, which is 4.44 in |
| Overall diameter (16 + 2 x 4.44) | 24.88 in, which is 632 mm |
| Circumference (pi x 24.88) | 78.16 in |
| Turns for one mile | 810.7 |
| Turns for one kilometer | 503.7 |
Comparing two sizes
When you change the tires, the important number is how the overall diameter changes. If it is bigger, the car travels farther on each turn of the wheel, and the speedometer, which counts the turns, reads too low. If it is smaller, the speedometer reads too high. The real speed is your reading times the new diameter divided by the old one.
change in diameter = new diameter / old diameter - 1 real speed = speedometer reading x new diameter / old diameter
Here we replace 205/55R16 with 225/45R17, a common "plus one" size, in which the wheel is one inch larger and the sidewall is lower:
| 205/55R16 | 225/45R17 | Difference | |
|---|---|---|---|
| Width | 205 mm (8.07 in) | 225 mm (8.86 in) | +20 mm |
| Sidewall | 4.44 in | 3.99 in | -0.45 in |
| Rim | 16 in | 17 in | +1 in |
| Overall diameter | 24.88 in | 24.97 in | +0.09 in |
| Turns per mile | 810.7 | 807.6 | -3.1 |
The overall diameter changes by only 0.38%. When the speedometer says 60, the real speed is 60.2. The new tire is wider and has a lower sidewall, but it has almost the same height, which is the goal of a plus-one change.
How much do other sizes change the height?
Compared with 205/55R16, and the real speed when the speedometer reads 60:
| Size | Overall diameter | Change | Real speed at 60 |
|---|---|---|---|
| 195/65R15 | 24.98 in | +0.41% | 60.2 |
| 205/55R16 (original) | 24.88 in | 0% | 60.0 |
| 225/45R17 | 24.97 in | +0.38% | 60.2 |
| 215/55R16 | 25.31 in | +1.74% | 61.0 |
| 235/45R17 | 25.33 in | +1.80% | 61.1 |
| 245/40R18 | 25.72 in | +3.37% | 62.0 |
| 225/50R17 | 25.86 in | +3.94% | 62.4 |
| 225/60R17 | 27.63 in | +11.06% | 66.6 |
| 265/70R17 | 31.61 in | +27.05% | 76.2 |
Chart: the overall diameter
195/65R15 ███████████████████████████████ 24.98 in 205/55R16 ███████████████████████████████ 24.88 in 225/45R17 ███████████████████████████████ 24.97 in 245/40R18 ████████████████████████████████ 25.72 in 225/60R17 ███████████████████████████████████ 27.63 in 265/70R17 ████████████████████████████████████████ 31.61 in
Chart: the change in speedometer reading
Green is within 3% of the original size, and red is over. A tire that is taller makes the speedometer read lower than your real speed.
225/45R17 █ +0.38% 195/65R15 █ +0.41% 215/55R16 ███ +1.74% 235/45R17 ███ +1.80% 245/40R18 █████ +3.37% 225/50R17 ██████ +3.94% 225/60R17 ████████████████ +11.06% 265/70R17 ████████████████████████████████████████ +27.05%
Why 3%?
Many car makers and tire shops suggest that the new size should be within about 3% of the original overall diameter. Beyond this, several things can go wrong:
- The speedometer and the odometer are wrong. At 3%, the real speed is 3% higher than what you see.
- The tire may rub on the fender or the suspension, especially when you turn or go over a bump.
- Traction control, ABS and the gearing are set for the original size, and may not work well.
- The load rating and the fit on the wheel may not be right.
The range for 205/55R16 is about 24.13 in to 25.62 in.
Tips
- Check the size of the wheel and the range of widths that it allows. A tire must fit the rim width.
- Check the load index and the speed rating of the new tires. They must be at least what the car maker says.
- For a plus-one change, raise the rim by one inch, widen the tire by about 10 to 20 mm, and lower the aspect ratio, until the diameter is nearly the same.
- Use the same size on all four wheels, unless the car maker gives different sizes for the front and rear.
- Winter tires are often a narrower size on a smaller wheel. Compare the diameter, as with any other change.
What this calculator does not include
- The change in the tire under load, and as it wears. A tire on a car is a little smaller than a new unloaded tire, and the real size changes as the tread wears.
- The width of the wheel and the fit inside the wheel arch.
- Load index, speed rating and the maker's approved sizes.
- Tire sizes given in inches, such as 31x10.50R15, which are worked out differently.
This tool gives an estimate for general information only. Check the fit and the safety of any new tire size with a tire shop or the vehicle manual.