255/75R20 vs 275/65R20
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/75R20 | 275/65R20 | Change |
|---|---|---|---|
| Overall height | 35.1" | 34.1" | -2.8 % |
| Sidewall | 7.53" | 7.04" | -6.5 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 110.1" | 107.0" | -2.8 % |
| Revs per mile | 575 | 592 | +2.8 % |
| Axle height | 17.53" | 17.04" | -12.5 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 255/75R20. Fit 275/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,719 on the road, so it over-counts by 281. At a true 70 mph the first turns 671 times a minute, the second 691.