295/65R18 vs 285/70R18
Going from one to the other changes the overall height by +1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/65R18 | 285/70R18 | Change |
|---|---|---|---|
| Overall height | 33.1" | 33.7" | +1.8 % |
| Sidewall | 7.55" | 7.85" | +4.0 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 104.0" | 105.9" | +1.8 % |
| Revs per mile | 609 | 598 | -1.8 % |
| Axle height | 16.55" | 16.85" | +7.8 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 295/65R18. Fit 285/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,184 on the road, so it under-counts by 184. At a true 70 mph the first turns 711 times a minute, the second 698.