Tire size comparison calculator
Put your current size on the left and the size you are considering on the right. The drawing is to scale; the tables tell you what changes on the road.
Metric (265/70R17) or inch (33x12.50R20). Axle ratio is used for the RPM table only.
275/70R18 is 1.10 in (28 mm) taller than 255/70R18 (+3.4%). With a speedometer calibrated for 255/70R18, an indicated 60 mph is really 62.1 mph. The diameter difference is more than ±3%, a range often quoted as a rule of thumb, so speedometer, gearing and clearance effects are larger.
Geometry only: this does not confirm that a size fits, is approved for or is safe on a particular vehicle. Wheel width and offset, suspension, clearances, load index, speed rating, the vehicle maker's specification and local regulations all apply — check with your owner's manual and a qualified tire professional.
| 255/70R18 | 275/70R18 | Difference | |
|---|---|---|---|
| Diameter | 32.06 in | 33.16 in | +1.10 in (+3.44%) |
| Diameter (mm) | 814 mm | 842 mm | +28 mm |
| Width | 10.04 in | 10.83 in | +0.79 in (+20 mm) |
| Sidewall | 7.03 in | 7.58 in | +0.55 in |
| Rim | 18 in | 18 in | ±0.0 in |
| Circumference | 100.7 in | 104.2 in | +3.46 in |
| Revs per mile | 629 | 608 | −21 |
| Ground clearance change | — | — | +0.55 in |
Speedometer error
| Speedometer shows | True speed on 275/70R18 | Difference |
|---|---|---|
| 20 mph | 20.7 mph | +0.7 mph |
| 30 mph | 31.0 mph | +1.0 mph |
| 40 mph | 41.4 mph | +1.4 mph |
| 50 mph | 51.7 mph | +1.7 mph |
| 60 mph | 62.1 mph | +2.1 mph |
| 70 mph | 72.4 mph | +2.4 mph |
| 80 mph | 82.8 mph | +2.8 mph |
| Speedometer shows | True speed on 275/70R18 | Difference |
|---|---|---|
| 30 km/h | 31.0 km/h | +1.0 km/h |
| 50 km/h | 51.7 km/h | +1.7 km/h |
| 80 km/h | 82.8 km/h | +2.8 km/h |
| 100 km/h | 103.4 km/h | +3.4 km/h |
| 120 km/h | 124.1 km/h | +4.1 km/h |
Assumes the speedometer was accurate on 255/70R18. Factory speedometers often read slightly high on purpose; check against GPS.
Engine RPM and gearing
| Road speed | 255/70R18 | 275/70R18 | Change |
|---|---|---|---|
| 45 mph | 1,760 rpm | 1,702 rpm | −58 rpm |
| 55 mph | 2,151 rpm | 2,080 rpm | −71 rpm |
| 65 mph | 2,542 rpm | 2,458 rpm | −84 rpm |
| 75 mph | 2,934 rpm | 2,836 rpm | −98 rpm |
To restore the original effective gearing on 275/70R18 you would need an axle ratio of about 3.86 (nearest ratios on our reference list: 3.73 or 3.91). For overdrive gears multiply the RPM by the gear ratio (e.g. ×0.69).
Other 18-inch sizes within ±3% of 255/70R18
| Size | Diameter | Width | vs original |
|---|---|---|---|
| 275/65R18 | 32.07 in | 10.83 in | +0.06% |
| 325/55R18 | 32.07 in | 12.80 in | +0.06% |
| 355/50R18 | 31.98 in | 13.98 in | −0.25% |
| 225/80R18 | 32.17 in | 8.86 in | +0.37% |
| 295/60R18 | 31.94 in | 11.61 in | −0.37% |
| 235/75R18 | 31.88 in | 9.25 in | −0.55% |
| 215/85R18 | 32.39 in | 8.46 in | +1.04% |
| 205/85R18 | 31.72 in | 8.07 in | −1.04% |
Computed combinations of standard widths and aspect ratios; not every size is manufactured. A similar diameter does not mean a size fits or is approved for your vehicle.
What to look at first
- Diameter difference (%) — the headline number. The ±3% often quoted as a rule of thumb is about speedometer and gearing effects; it does not tell you whether a size fits.
- Speedometer table — a taller tire makes the speedometer read low. Your true speed is higher than what the dash shows.
- Width and sidewall — extra width reduces clearance to suspension and body parts; sidewall height changes how much the tire flexes.
- RPM — taller tires lower engine speed at a given road speed, which feels like a taller (numerically lower) axle ratio.
Pre-computed comparisons
- 265/70R17 vs 285/70R17
- 245/75R16 vs 265/75R16
- 265/70R16 vs 245/75R16
- 275/65R18 vs 275/70R18
- 265/65R18 vs 275/65R18
- 265/75R16 vs 285/75R16
- 285/70R17 vs 315/70R17
- 275/60R20 vs 275/65R20
- 265/70R17 vs 275/65R18
- 225/65R17 vs 225/60R18
- 265/75R15 vs 31x10.50R15
- 285/70R17 vs 33x12.50R17
- 315/70R17 vs 35x12.50R17
Frequently asked questions
What does the ±3% figure mean?
It is a rule of thumb often quoted in the tire trade for how far the overall diameter can differ before speedometer, gearing and clearance effects become significant. It is not a legal limit and not a fitment approval: a size within 3% can still be unsuitable (for example because of width, offset, load index or speed rating), and the vehicle maker's specification and local regulations take precedence.
Is a wider tire always taller?
Not necessarily. With the same aspect ratio a wider tire is also taller, because the sidewall is a percentage of the width (265/70 vs 285/70). With a lower aspect ratio it can be the same height or shorter — 225/45R17 is wider than 205/55R16 but only 0.09 in taller. Compare diameters, not widths.
Why does the comparison mention ground clearance?
The axle sits at the center of the wheel, so it rises by half of the diameter increase. A tire 1.10 in taller lifts the axle and differential by about 0.55 in.
What axle ratio should I enter?
Your differential (ring-and-pinion) ratio, often on the axle tag or in the build sheet — for example 3.21, 3.73 or 4.10. It only affects the RPM table; leave 3.73 if you are not sure.
Can I compare a metric size with an inch size?
Yes. Enter for example 285/70R17 and 33x12.50R17; both are converted to inches and millimeters before comparing.
Related tools
- Speedometer error calculator
How fast you are really going after a tire change.
- Gear ratio & RPM
Engine RPM and the regear needed for bigger tires.
- Plus size calculator
Sizes that keep your diameter on a bigger or smaller wheel.
- Inch ↔ metric converter
33x12.50R17 to metric and back, with closest equivalents.