275/40R18 vs 285/40R18Tire Size Comparison

Safe

The 1.2% diameter difference is within the accepted 3% tolerance.

275/40R18677mm dia285/40R18685mm dia+4.0mm

Comparison Details

Overall Diameter677.2 vs 685.2 mm (+8.0 mm)
Diameter Difference+1.18%
Section Width275 vs 285 mm (+10 mm)
Sidewall Height110.0 vs 114.0 mm (+4.0 mm)
Circumference2127.5 vs 2152.6 mm
Clearance Change+4.0 mm
Speedometer Error+1.18%
Revs/Mile756 vs 748

Speedometer Correction

When switching from 275/40R18 to 285/40R18, your speedometer will differ from actual speed.

SpeedometerActual SpeedDifference
30 km/h30.4 km/h+0.4 km/h
40 km/h40.5 km/h+0.5 km/h
50 km/h50.6 km/h+0.6 km/h
60 km/h60.7 km/h+0.7 km/h
70 km/h70.8 km/h+0.8 km/h
80 km/h80.9 km/h+0.9 km/h
90 km/h91.1 km/h+1.1 km/h
100 km/h101.2 km/h+1.2 km/h
110 km/h111.3 km/h+1.3 km/h
120 km/h121.4 km/h+1.4 km/h
130 km/h131.5 km/h+1.5 km/h

Individual Tire Size Details

275/40R18
Full specifications, compatible vehicles, and alternatives
285/40R18
Full specifications, compatible vehicles, and alternatives

Practical Guidance

Switching from 275/40R18 to 285/40R18 is within the industry-accepted 3% diameter tolerance. Your speedometer, ABS, and traction control should function normally. Both sizes fit the same 18-inch rim, so no wheel change is needed.

The 3% diameter tolerance is a widely accepted industry guideline referenced by the ETRTO and most vehicle manufacturers. When in doubt, consult your vehicle's owner's manual or a qualified tire professional. For U.S. tire safety standards and recalls, see the NHTSA Tire Safety page.

Frequently Asked Questions

Safe. The overall diameter differs by 1.18%, width changes by 10mm.

Speedometer error will be 1.18%. At an indicated 100 km/h, actual speed is about 101.2 km/h.

Ground clearance changes by +4.0mm per side.