255/35R20 vs 275/30R20
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/35R20 | 275/30R20 | Change |
|---|---|---|---|
| Overall height | 27.0" | 26.5" | -2.0 % |
| Sidewall | 3.51" | 3.25" | -7.6 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 84.9" | 83.2" | -2.0 % |
| Revs per mile | 746 | 761 | +2.0 % |
| Axle height | 13.51" | 13.25" | -6.8 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 255/35R20. Fit 275/30R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,803 on the road, so it over-counts by 197. At a true 70 mph the first turns 871 times a minute, the second 888.
What drivers reported
255/35R20 carries 4 complaints. Nobody has filed one naming 275/30R20.