How two tyre sizes are compared
Every figure comes from the three numbers on the sidewall: section width, profile and rim diameter.
diameter = (width × profile ÷ 100) × 2 + rim × 25.4, and circumference = π × diameter
- The sidewall height is the section width multiplied by the profile percentage, so a 225/45 tyre has 101.3 mm of sidewall.
- Overall diameter is twice that sidewall plus the rim, with the rim converted from inches to millimetres at 25.4 mm per inch.
- Circumference is π times diameter, and that sets how far the wheel travels in one turn. It decides your speedometer reading and how many times the wheel turns per kilometre.
- Ground clearance moves by half the diameter difference, because the extra height splits between the top and the bottom of the wheel.
Worked example — 225/45R17 measures 634.3 mm across and 235/40R18 measures 645.2 mm, so the 18 inch tyre is 10.9 mm (1.7%) taller.
Common tyre sizes measured
| Size | Diameter | Circumference | Revs per km | Sidewall |
|---|---|---|---|---|
| 175/65R14 | 583.1 mm | 1,832 mm | 546 | 113.8 mm |
| 185/65R15 | 621.5 mm | 1,952 mm | 512 | 120.3 mm |
| 195/65R15 | 634.5 mm | 1,993 mm | 502 | 126.8 mm |
| 205/55R16 | 631.9 mm | 1,985 mm | 504 | 112.8 mm |
| 215/60R16 | 664.4 mm | 2,087 mm | 479 | 129 mm |
| 225/45R17 | 634.3 mm | 1,993 mm | 502 | 101.3 mm |
| 235/60R18 | 739.2 mm | 2,322 mm | 431 | 141 mm |
| 245/40R19 | 678.6 mm | 2,132 mm | 469 | 98 mm |
| 265/65R17 | 776.3 mm | 2,439 mm | 410 | 172.3 mm |
| 33x12.5R15 | 838.2 mm | 2,633 mm | 380 | 228.6 mm |
Swipe the table sideways to see the rest.
Diameter and circumference are worked out from the size written on the sidewall.
Common swaps compared
| Tyre A | Tyre B | Diameter difference | Fit check | Speedometer at 100 km/h |
|---|---|---|---|---|
| 195/65R15 | 205/60R15 | -7.5 mm (-1.2%) | Usually a straight swap | 98.8 km/h actual |
| 205/55R16 | 215/60R16 | +32.5 mm (+5.1%) | Too big a change to fit safely | 105.1 km/h actual |
| 225/45R17 | 235/40R18 | +10.9 mm (+1.7%) | Check clearance and your speedometer | 101.7 km/h actual |
| 235/65R17 | 265/70R17 | +65.5 mm (+8.9%) | Too big a change to fit safely | 108.9 km/h actual |
| 265/70R17 | 285/70R17 | +28 mm (+3.5%) | Too big a change to fit safely | 103.5 km/h actual |
Swipe the table sideways to see the rest.
Tyre A is the size you are used to, so its speedometer reading is taken as correct.
Things to consider
- Check the tyre placard or the owner's manual before you buy. It lists the sizes the maker approves, and the load and speed ratings you should not go below.
- A larger diameter can rub on the wheel arch or the suspension guard at full lock or full bump. Turn the wheel and check by hand before you rely on it.
- Keep the load and speed index at or above the original. A bigger tyre that carries less is not an upgrade.
- If the diameter changes by more than about 1.5%, the speedometer and odometer read low. Check whether that matters where you are.
Frequently asked questions
How much difference is safe when I change tyre size?
Within about 1.5% of the original diameter usually needs no other changes. Between 1.5% and 3% you should check clearance, the speedometer reading and whether the width still suits the rim.
Why does my speedometer read wrong after a size change?
Speed is worked out from how fast the wheel turns. A larger circumference covers more ground per turn, so your true speed is higher than the reading unless the car is recalibrated.
Does a bigger tyre make the car faster?
No. A taller tyre covers slightly more ground per turn but needs more torque to turn it, so acceleration and fuel use usually get a little worse.
What if my size has no profile number, like 175R14?
Older sizes are written without one. This calculator reads those as 82 series, so the sidewall is taken as 82% of the width.
Is this the same as a tire size comparison?
Yes. Tyre and tire are the same word, so the figures here apply whichever spelling you use.
Results are estimates for general information only, not financial, medical or professional advice.