275/65R20 vs 265/70R20
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 | 275/65R20 | 265/70R20 | Change |
|---|---|---|---|
| Overall height | 34.1" | 34.6" | +1.6 % |
| Sidewall | 7.04" | 7.30" | +3.8 % |
| Section width | 275 mm | 265 mm | -3.6 % |
| Circumference | 107.0" | 108.7" | +1.6 % |
| Revs per mile | 592 | 583 | -1.6 % |
| Axle height | 17.04" | 17.30" | +6.8 mm |
| Approved rim width | 7.5–10" | 7.5–9.5" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" 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 275/65R20. Fit 265/70R20 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 10,156 on the road, so it under-counts by 156. At a true 70 mph the first turns 691 times a minute, the second 680.