285/65R20 vs 265/70R20
Going from one to the other changes the overall height by +0.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/65R20 | 265/70R20 | Change |
|---|---|---|---|
| Overall height | 34.6" | 34.6" | +0.1 % |
| Sidewall | 7.29" | 7.30" | +0.1 % |
| Section width | 285 mm | 265 mm | -7.0 % |
| Circumference | 108.7" | 108.7" | +0.1 % |
| Revs per mile | 583 | 583 | -0.1 % |
| Axle height | 17.29" | 17.30" | +0.3 mm |
| Approved rim width | 8–10.5" | 7.5–9.5" | overlap 8–9.5" |
Both are approved on a rim 8" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 285/65R20. Fit 265/70R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | +0.0 |
| 50 mph | 50.0 mph | +0.0 |
| 70 mph | 70.0 mph | +0.0 |
| 90 mph | 90.1 mph | +0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,006 on the road, so it under-counts by 6. At a true 70 mph the first turns 680 times a minute, the second 680.