285/70R18 vs 275/75R18
Going from one to the other changes the overall height by +1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/70R18 | 275/75R18 | Change |
|---|---|---|---|
| Overall height | 33.7" | 34.2" | +1.6 % |
| Sidewall | 7.85" | 8.12" | +3.4 % |
| Section width | 285 mm | 275 mm | -3.5 % |
| Circumference | 105.9" | 107.6" | +1.6 % |
| Revs per mile | 598 | 589 | -1.6 % |
| Axle height | 16.85" | 17.12" | +6.8 mm |
| Approved rim width | 8–10.5" | 7.5–10" | 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 285/70R18. Fit 275/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,158 on the road, so it under-counts by 158. At a true 70 mph the first turns 698 times a minute, the second 687.