235/65R17 vs 235/55R18
235/55R18 is −0.85″ in overall diameter and 0 mm in section width against 235/65R17 — a diameter change of −2.93%, which stays inside the commonly used ±3% diameter guide. An indicated 60 mph would really be 58.2 mph.
Opens the calculator prefilled. The pair stays in the link fragment, not an account or server log.
Every dimension
| Dimension | 235/65R17 → 235/55R18 | Change |
|---|---|---|
| Overall diameter | 29.03″ → 28.18″ | −0.85″ (−2.93%) |
| Section width | 235 mm → 235 mm | 0 mm (0.00%) |
| Sidewall height | 152.8 mm → 129.3 mm | −23.5 mm |
| Rolling circumference | 91.19″ → 88.52″ | −2.67″ |
| Revolutions per mile | 694.8 → 715.8 | +21.0 |
| Axle height / clearance | 14.51″ → 14.09″ | −0.43″ |
Which is taller, 235/65R17 or 235/55R18?
235/65R17 is the taller of the two, at 29.03″ against 28.18″. The gap is 0.85 inches, or 21.6 mm.
Both are metric sizes, so neither states its height directly. Both figures below are derived from the section width, aspect ratio and wheel diameter in the code. 235/65R17 is 235 mm wide with a 152.8 mm sidewall on a 17″ wheel; 235/55R18 is 235 mm wide with a 129.3 mm sidewall on a 18″ wheel.
What the speedometer will read
| Speedometer reads | Actual speed | Error |
|---|---|---|
| 25 mph | 24.3 mph | −0.7 mph |
| 40 mph | 38.8 mph | −1.2 mph |
| 55 mph | 53.4 mph | −1.6 mph |
| 60 mph | 58.2 mph | −1.8 mph |
| 70 mph | 67.9 mph | −2.1 mph |
| 80 mph | 77.7 mph | −2.3 mph |
Fitting 235/55R18 in place of 235/65R17 multiplies every speed and distance reading by 0.97070. Over 10,000 indicated miles the odometer is out by −293 miles.
Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 235/65R17 would need 3.62 to keep the same engine speed on 235/55R18. Work it out for your own axle ratio.
Full specification of each size
235/65R17
| Dimension | Metric | Imperial |
|---|---|---|
| Section width | 235.0 mm | 9.25″ |
| Sidewall height | 152.8 mm | 6.01″ |
| Wheel diameter | 431.8 mm | 17.0″ |
| Overall diameter | 737.3 mm | 29.03″ |
| Rolling circumference | 2,316.3 mm | 91.19″ |
| Revolutions per mile | 431.7 / km | 694.8 / mile |
| Measuring rim (estimate) | 178 mm | 7″ |
235/55R18
| Dimension | Metric | Imperial |
|---|---|---|
| Section width | 235.0 mm | 9.25″ |
| Sidewall height | 129.3 mm | 5.09″ |
| Wheel diameter | 457.2 mm | 18.0″ |
| Overall diameter | 715.7 mm | 28.18″ |
| Rolling circumference | 2,248.4 mm | 88.52″ |
| Revolutions per mile | 444.8 / km | 715.8 / mile |
| Measuring rim (estimate) | 191 mm | 7.5″ |
Rim widths differ too: 235/65R17 is designed for 6.5″–8″ and 235/55R18 for 7″–8.5″. Their estimated measuring rims differ; confirm the exact tire maker's approved range before choosing a wheel.
Can you swap one for the other?
Inside the ±3% diameter guide
The diameter difference of −2.93% is within the commonly used ±3% geometry guideline, which limits the change to speedometer, odometer and effective gearing calculations. It is not a fitment or safety approval. The section width difference of 0 mm is separate, and physical clearance still depends on the wheel and vehicle.
Neither figure says anything about physical fitment. Whether 235/55R18 clears the strut, the fender liner and the steering stops depends on wheel offset and suspension geometry that a size code does not encode. For maintenance rather than sizing, Car Basics covers tire care.
235/65R17 vs 235/55R18: common questions
- Is 235/55R18 bigger than 235/65R17?
- 235/55R18 is 0.85 inches shorter and 0 mm wider. In overall diameter that is −2.93%, which stays inside the commonly used ±3% diameter guideline.
- How much will 235/55R18 throw off my speedometer?
- Every reading scales by 0.97070. An indicated 60 mph becomes 58.2 mph, and every 100 miles on the odometer is really 97.1 miles.
- How much does 235/55R18 change ground clearance over 235/65R17?
- The axle centerline moves by 0.43 inches — half the diameter change, because half the extra height sits below the hub and half above it.
- Will 235/55R18 fit where 235/65R17 fits?
- Nothing derived from the size code can answer that. Clearance depends on wheel offset, suspension travel and body dimensions that no tire size contains. This page tells you exactly what changes dimensionally, which is the part that can be calculated.
Related comparisons
Other tire maths on this site
- tire size calculatorRead any tire size code and get overall diameter, sidewall height, circumference and revolutions per mile, with the arithmetic shown.
- tire size comparisonPut two sizes side by side and see the difference in diameter, width, sidewall, ground clearance and speedometer reading.
- tire size chartEvery common size sorted by wheel diameter, with overall diameter in inches and millimeters and revolutions per mile.
- tire diameter calculatorOverall diameter, radius and rolling circumference from the size code, and the reverse — the sizes that produce a diameter you want.
- speedometer error calculatorHow far out your speedometer and odometer read after a tire size change, at any indicated speed.
- plus sizing calculatorLarger wheel, same rolling diameter. Plus one and plus two candidates ranked by how close they hold the original diameter.
- gear ratio calculatorThe axle ratio that restores stock engine rpm after a tire diameter change, plus cruising rpm at any speed.
- metric to inch tire conversionConvert a metric size such as 285/70R17 to its inch equivalent, and an inch size such as 33x12.50R17 back to metric.
- rim width rangeThe rim widths a size is designed to mount on, and how section width changes as the rim gets wider.
- tire size meaningWhat every character in 265/70R17 stands for, digit by digit, including the load index and speed rating.