255/70R20 vs 275/65R20
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 | 255/70R20 | 275/65R20 | Change |
|---|---|---|---|
| Overall height | 34.1" | 34.1" | +0.1 % |
| Sidewall | 7.03" | 7.04" | +0.1 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 107.0" | 107.0" | +0.1 % |
| Revs per mile | 592 | 592 | -0.1 % |
| Axle height | 17.03" | 17.04" | +0.3 mm |
| Approved rim width | 7–9.5" | 7.5–10" | 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 255/70R20. Fit 275/65R20 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 691 times a minute, the second 691.
What drivers reported
Nobody has filed a complaint naming 255/70R20. 275/65R20 carries 9.