255/40R18 dimensions
| Measure | Inches | Millimetres |
|---|---|---|
| Overall height | 26.03 in | 661 mm |
| Section width | 10.04 in | 255 mm |
| Sidewall height | 4.02 in | 102.0 mm |
| Wheel diameter | 18 in | 457 mm |
| Circumference | 81.8 in | 2,077 mm |
| Revolutions per mile | 774.8 |
The math
| Step | Math | Result |
|---|---|---|
| Sidewall | 255 × 40% = 102.0 mm ÷ 25.4 | 4.02 in |
| Overall height | 18 + 2 × 4.02 | 26.03 in |
| Width | 255 mm ÷ 25.4 | 10.04 in |
| Circumference | π × 26.03 | 81.8 in |
| Revs per mile | 63,360 ÷ 81.8 | 774.8 |
With a 4.0 in sidewall this is a medium sidewall: a balance of cushion and response. Real tires vary by brand and model, and the loaded radius under the vehicle is a bit smaller than this unloaded figure.
One step either way on a 18 in wheel
Each row changes one thing and shows the height change against 255/40R18, plus what 60 mph on the speedometer would really be if you swapped to that size.
| Change | Size | Height | Height change | 60 mph shows as | Verdict |
|---|---|---|---|---|---|
| Wider | 265/40R18 (compare) | 26.3 in | +1.2% | 60.7 mph | Close match |
| Narrower | 245/40R18 (compare) | 25.7 in | -1.2% | 59.3 mph | Close match |
| Taller sidewall | 255/45R18 (compare) | 27.0 in | +3.9% | 62.3 mph | Check clearance |
| Shorter sidewall | 255/35R18 (compare) | 25.0 in | -3.9% | 57.7 mph | Check clearance |
| Wider, shorter sidewall | 265/35R18 (compare) | 25.3 in | -2.8% | 58.3 mph | Close match |
| Narrower, taller sidewall | 245/45R18 (compare) | 26.7 in | +2.5% | 61.5 mph | Close match |
Calculated sizes within ±3% on other wheels
Plus and minus sizes that keep the overall height within about 3% (a rule of thumb from Discount Tire and 1010tires), widths within 30 mm. Calculated, not a list of tires on sale: check that a tire exists in that size.
17 in wheels (one wheel size smaller)
| Size | Height | Height change | Width change |
|---|---|---|---|
| 255/45R17 (compare) | 26.0 in | +0.0% | +0 mm |
| 285/40R17 (compare) | 26.0 in | -0.2% | +30 mm |
| 225/50R17 (compare) | 25.9 in | -0.7% | -30 mm |
| 235/50R17 (compare) | 26.3 in | +0.8% | -20 mm |
19 in wheels (one wheel size larger)
| Size | Height | Height change | Width change |
|---|---|---|---|
| 255/35R19 (compare) | 26.0 in | -0.0% | +0 mm |
| 225/40R19 (compare) | 26.1 in | +0.2% | -30 mm |
| 265/35R19 (compare) | 26.3 in | +1.0% | +10 mm |
| 245/35R19 (compare) | 25.8 in | -1.1% | -10 mm |
20 in wheels (two wheel sizes larger)
| Size | Height | Height change | Width change |
|---|---|---|---|
| 255/30R20 (compare) | 26.0 in | -0.0% | +0 mm |
| 225/35R20 (compare) | 26.2 in | +0.7% | -30 mm |
| 265/30R20 (compare) | 26.3 in | +0.9% | +10 mm |
| 245/30R20 (compare) | 25.8 in | -0.9% | -10 mm |
Try it with your numbers
Compare 255/40R18 with another size · Convert to metric or find a 33 in size · Sizes that fit my wheel
Calculated from the size alone. Real tires vary a little by brand and model, and the loaded radius is a bit smaller. Confirm the wheel width range, clearance, load index and speed rating for your vehicle. See how to read a tire size, the load index chart and the speed rating chart.
Browse: all sizes in inches · 255/35R18 · 255/45R18.