295/70R18 vs 285/70R18
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 | 295/70R18 | 285/70R18 | Change |
|---|---|---|---|
| Overall height | 34.3" | 33.7" | -1.6 % |
| Sidewall | 8.13" | 7.85" | -3.4 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 107.6" | 105.9" | -1.6 % |
| Revs per mile | 589 | 598 | +1.6 % |
| Axle height | 17.13" | 16.85" | -7.0 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 295/70R18. Fit 285/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,839 on the road, so it over-counts by 161. At a true 70 mph the first turns 687 times a minute, the second 698.