255/30R20 vs 275/30R20
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 | 255/30R20 | 275/30R20 | Change |
|---|---|---|---|
| Overall height | 26.0" | 26.5" | +1.8 % |
| Sidewall | 3.01" | 3.25" | +7.8 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 81.8" | 83.2" | +1.8 % |
| Revs per mile | 775 | 761 | -1.8 % |
| Axle height | 13.01" | 13.25" | +6.0 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 255/30R20. Fit 275/30R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,182 on the road, so it under-counts by 182. At a true 70 mph the first turns 904 times a minute, the second 888.