TireGeoCalcTyre Size Comparison for Common UK Size Swaps
Quick answer
Nominal geometryTable example: 205/55R16 → 225/45R17 · original-size calibration assumed
- 225/45R17 diameter
- 24.97in
- Diameter change
- +0.38%
- Actual at indicated 60 mph
- 60.23mph
Calculated from size markings. Not measured rolling dimensions or vehicle fitment approval.
A tyre size comparison starts with overall diameter, including both the wheel and its sidewalls. This chart groups familiar UK tyre size combinations into useful starting points for comparison. The rows give nominal dimensions, not fitment approval for a particular car. Read each candidate against the original size shown in its row, then check the vehicle documentation before choosing a tyre.
Common UK tyre sizes and comparison candidates
Each group has its own reference tyre. The original row shows zero change; the following rows compare with that same reference. Read across each row to follow changes in sidewall height, circumference and speed.
| Tire size | Reference size | Diameter / height (mm) | Diameter / height (in) | Sidewall (mm) | Circumference (mm) | Revolutions / mile | Diameter change (%) | Actual at indicated 60 mph | Actual minus indicated (mph) | Manual check |
|---|---|---|---|---|---|---|---|---|---|---|
| 205/55R16 | 205/55R16 | 631.9 | 24.88 | 112.8 | 1985.2 | 810.7 | 0.00% | 60.00 mph | 0.00 mph | |
| 225/45R17 | 205/55R16 | 634.3 | 24.97 | 101.3 | 1992.7 | 807.6 | +0.38% | 60.23 mph | +0.23 mph | |
| 225/40R18 | 205/55R16 | 637.2 | 25.09 | 90.0 | 2001.8 | 803.9 | +0.84% | 60.50 mph | +0.50 mph | |
| 195/65R15 | 195/65R15 | 634.5 | 24.98 | 126.8 | 1993.3 | 807.4 | 0.00% | 60.00 mph | 0.00 mph | |
| 205/55R16 | 195/65R15 | 631.9 | 24.88 | 112.8 | 1985.2 | 810.7 | -0.41% | 59.75 mph | -0.25 mph | |
| 225/45R17 | 195/65R15 | 634.3 | 24.97 | 101.3 | 1992.7 | 807.6 | -0.03% | 59.98 mph | -0.02 mph | |
| 215/55R17 | 215/55R17 | 668.3 | 26.31 | 118.3 | 2099.5 | 766.5 | 0.00% | 60.00 mph | 0.00 mph | |
| 235/45R18 | 215/55R17 | 668.7 | 26.33 | 105.8 | 2100.8 | 766.1 | +0.06% | 60.04 mph | +0.04 mph | |
| 225/50R18 | 215/55R17 | 682.2 | 26.86 | 112.5 | 2143.2 | 750.9 | +2.08% | 61.25 mph | +1.25 mph | |
| 235/55R18 | 235/55R18 | 715.7 | 28.18 | 129.3 | 2248.4 | 715.8 | 0.00% | 60.00 mph | 0.00 mph | |
| 235/50R19 | 235/55R18 | 717.6 | 28.25 | 117.5 | 2254.4 | 713.9 | +0.27% | 60.16 mph | +0.16 mph | |
| 245/45R20 | 235/55R18 | 728.5 | 28.68 | 110.3 | 2288.7 | 703.2 | +1.79% | 61.07 mph | +1.07 mph | |
| 235/45R20 | 235/55R18 | 719.5 | 28.33 | 105.8 | 2260.4 | 712.0 | +0.53% | 60.32 mph | +0.32 mph |
Nominal dimensions calculated from tire size markings; rounding is for display only.
- Tire size
- 205/55R16
- Reference size
- 205/55R16
- Diameter / height (mm)
- 631.9
- Diameter / height (in)
- 24.88
- Sidewall (mm)
- 112.8
- Circumference (mm)
- 1985.2
- Revolutions / mile
- 810.7
- Diameter change (%)
- 0.00%
- Actual at indicated 60 mph
- 60.00 mph
- Actual minus indicated (mph)
- 0.00 mph
- Manual check
- Tire size
- 225/45R17
- Reference size
- 205/55R16
- Diameter / height (mm)
- 634.3
- Diameter / height (in)
- 24.97
- Sidewall (mm)
- 101.3
- Circumference (mm)
- 1992.7
- Revolutions / mile
- 807.6
- Diameter change (%)
- +0.38%
- Actual at indicated 60 mph
- 60.23 mph
- Actual minus indicated (mph)
- +0.23 mph
- Manual check
- Tire size
- 225/40R18
- Reference size
- 205/55R16
- Diameter / height (mm)
- 637.2
- Diameter / height (in)
- 25.09
- Sidewall (mm)
- 90.0
- Circumference (mm)
- 2001.8
- Revolutions / mile
- 803.9
- Diameter change (%)
- +0.84%
- Actual at indicated 60 mph
- 60.50 mph
- Actual minus indicated (mph)
- +0.50 mph
- Manual check
- Tire size
- 195/65R15
- Reference size
- 195/65R15
- Diameter / height (mm)
- 634.5
- Diameter / height (in)
- 24.98
- Sidewall (mm)
- 126.8
- Circumference (mm)
- 1993.3
- Revolutions / mile
- 807.4
- Diameter change (%)
- 0.00%
- Actual at indicated 60 mph
- 60.00 mph
- Actual minus indicated (mph)
- 0.00 mph
- Manual check
- Tire size
- 205/55R16
- Reference size
- 195/65R15
- Diameter / height (mm)
- 631.9
- Diameter / height (in)
- 24.88
- Sidewall (mm)
- 112.8
- Circumference (mm)
- 1985.2
- Revolutions / mile
- 810.7
- Diameter change (%)
- -0.41%
- Actual at indicated 60 mph
- 59.75 mph
- Actual minus indicated (mph)
- -0.25 mph
- Manual check
- Tire size
- 225/45R17
- Reference size
- 195/65R15
- Diameter / height (mm)
- 634.3
- Diameter / height (in)
- 24.97
- Sidewall (mm)
- 101.3
- Circumference (mm)
- 1992.7
- Revolutions / mile
- 807.6
- Diameter change (%)
- -0.03%
- Actual at indicated 60 mph
- 59.98 mph
- Actual minus indicated (mph)
- -0.02 mph
- Manual check
- Tire size
- 215/55R17
- Reference size
- 215/55R17
- Diameter / height (mm)
- 668.3
- Diameter / height (in)
- 26.31
- Sidewall (mm)
- 118.3
- Circumference (mm)
- 2099.5
- Revolutions / mile
- 766.5
- Diameter change (%)
- 0.00%
- Actual at indicated 60 mph
- 60.00 mph
- Actual minus indicated (mph)
- 0.00 mph
- Manual check
- Tire size
- 235/45R18
- Reference size
- 215/55R17
- Diameter / height (mm)
- 668.7
- Diameter / height (in)
- 26.33
- Sidewall (mm)
- 105.8
- Circumference (mm)
- 2100.8
- Revolutions / mile
- 766.1
- Diameter change (%)
- +0.06%
- Actual at indicated 60 mph
- 60.04 mph
- Actual minus indicated (mph)
- +0.04 mph
- Manual check
- Tire size
- 225/50R18
- Reference size
- 215/55R17
- Diameter / height (mm)
- 682.2
- Diameter / height (in)
- 26.86
- Sidewall (mm)
- 112.5
- Circumference (mm)
- 2143.2
- Revolutions / mile
- 750.9
- Diameter change (%)
- +2.08%
- Actual at indicated 60 mph
- 61.25 mph
- Actual minus indicated (mph)
- +1.25 mph
- Manual check
- Tire size
- 235/55R18
- Reference size
- 235/55R18
- Diameter / height (mm)
- 715.7
- Diameter / height (in)
- 28.18
- Sidewall (mm)
- 129.3
- Circumference (mm)
- 2248.4
- Revolutions / mile
- 715.8
- Diameter change (%)
- 0.00%
- Actual at indicated 60 mph
- 60.00 mph
- Actual minus indicated (mph)
- 0.00 mph
- Manual check
- Tire size
- 235/50R19
- Reference size
- 235/55R18
- Diameter / height (mm)
- 717.6
- Diameter / height (in)
- 28.25
- Sidewall (mm)
- 117.5
- Circumference (mm)
- 2254.4
- Revolutions / mile
- 713.9
- Diameter change (%)
- +0.27%
- Actual at indicated 60 mph
- 60.16 mph
- Actual minus indicated (mph)
- +0.16 mph
- Manual check
- Tire size
- 245/45R20
- Reference size
- 235/55R18
- Diameter / height (mm)
- 728.5
- Diameter / height (in)
- 28.68
- Sidewall (mm)
- 110.3
- Circumference (mm)
- 2288.7
- Revolutions / mile
- 703.2
- Diameter change (%)
- +1.79%
- Actual at indicated 60 mph
- 61.07 mph
- Actual minus indicated (mph)
- +1.07 mph
- Manual check
- Tire size
- 235/45R20
- Reference size
- 235/55R18
- Diameter / height (mm)
- 719.5
- Diameter / height (in)
- 28.33
- Sidewall (mm)
- 105.8
- Circumference (mm)
- 2260.4
- Revolutions / mile
- 712.0
- Diameter change (%)
- +0.53%
- Actual at indicated 60 mph
- 60.32 mph
- Actual minus indicated (mph)
- +0.32 mph
- Manual check
Calculated dimensions are nominal, not measured rolling dimensions or vehicle fitment approval.
Read the three parts of a tyre size together
In 205/55R16, the first number is section width in millimetres. The second is sidewall height as a percentage of that width; the final number is wheel diameter in inches. R describes radial construction. Width and wheel diameter also affect height.
Calculate one sidewall by multiplying width by aspect ratio, then add two sidewalls to the wheel diameter. Circumference follows from total diameter. The calculation describes an ideal circle. Brand, tread, measuring rim, pressure and load affect published dimensions; look up the exact specifications.
Compare the common car combinations
Start with the 205/55R16 reference when investigating 225/45R17 or 225/40R18. A larger wheel occupies more of the assembly while a shorter sidewall offsets much of that increase. Read both columns: close overall height can conceal a substantial sidewall change. Wider candidates need more lateral space.
The 195/65R15 group shows the same process from a smaller wheel. Compare each candidate directly with 195/65R15; adding rounded percentages from another group introduces error because percentage change depends on the starting diameter. Calculate different original axle sizes separately.
Use a separate reference for taller combinations
The 215/55R17 and 235/55R18 groups are separate from the smaller car examples. Sharing a wheel diameter does not make their overall heights similar. Choose the group matching your original size and follow its reference column. If that original is absent, enter it in the calculator to obtain the correct percentage.
Wheel width, offset, brake clearance, load specification and the tyre manufacturer's approved rim range are separate checks. The taller groups illustrate why these details matter: changing wheel diameter and section width together can preserve overall height while changing the space occupied around the suspension, brakes and body.
Interpret the speed column in the right direction
The speed column estimates actual speed at an indicated 60 mph, assuming calibration to the original tyre with no other instrument error. A taller candidate covers more distance per revolution, increasing calculated actual speed; a shorter one reduces it. Actual minus indicated gives the difference in mph, including its direction.
With a taller tyre, the speedometer can read below actual speed; UK/EU rules prohibit under-reading, so check the vehicle manual even below 3%. These nominal calculations do not approve fitment: confirm the vehicle and product specifications, and check the manual for any change exceeding 3%. Use the calculator below for your exact pair.
Frequently asked questions
Is tyre different from tire in a size calculation?
No. Tyre is the UK spelling; tire is used elsewhere on this en-US site. A metric size code is calculated the same way with either spelling. The spelling does not change wheel diameter, aspect ratio or circumference.
Does a close diameter prove that a swap fits?
No. Close diameter only describes overall height. Wheel width, offset, brake clearance and the exact tyre specifications determine other parts of the package, so two similarly tall assemblies can occupy different spaces.
Can I reuse a 16-inch tyre on a 17-inch wheel?
No. The wheel diameter in the tyre marking must match the wheel. Plus sizing means selecting a different tyre size for the different wheel, then comparing the resulting overall diameter.
Why do two similar sizes show different revs per mile?
Revolutions per mile are calculated from circumference. A larger nominal circle requires fewer revolutions to cover the same distance. Actual rolling data from a manufacturer may differ from this geometric estimate.