295/30R18 vs 285/30R18
Going from one to the other changes the overall height by -0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/30R18 | 285/30R18 | Change |
|---|---|---|---|
| Overall height | 25.0" | 24.7" | -0.9 % |
| Sidewall | 3.48" | 3.37" | -3.4 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 78.4" | 77.7" | -0.9 % |
| Revs per mile | 808 | 815 | +0.9 % |
| Axle height | 12.48" | 12.37" | -3.0 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 295/30R18. Fit 285/30R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,905 on the road, so it over-counts by 95. At a true 70 mph the first turns 942 times a minute, the second 951.