275/40R18 vs 285/40R18
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 275/40R18 | 285/40R18 | Change |
|---|---|---|---|
| Overall height | 26.7" | 27.0" | +1.2 % |
| Sidewall | 4.33" | 4.49" | +3.6 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 83.8" | 84.7" | +1.2 % |
| Revs per mile | 756 | 748 | -1.2 % |
| Axle height | 13.33" | 13.49" | +4.0 mm |
| Approved rim width | 7.5–10" | 8–10.5" | overlap 8–10" |
Both are approved on a rim 8" to 10" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 275/40R18. Fit 285/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,118 on the road, so it under-counts by 118. At a true 70 mph the first turns 883 times a minute, the second 872.
What drivers reported
275/40R18 carries 2 complaints. Nobody has filed one naming 285/40R18.