275/30R20 vs 255/35R20
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 | 275/30R20 | 255/35R20 | Change |
|---|---|---|---|
| Overall height | 26.5" | 27.0" | +2.0 % |
| Sidewall | 3.25" | 3.51" | +8.2 % |
| Section width | 275 mm | 255 mm | -7.3 % |
| Circumference | 83.2" | 84.9" | +2.0 % |
| Revs per mile | 761 | 746 | -2.0 % |
| Axle height | 13.25" | 13.51" | +6.8 mm |
| Approved rim width | 7.5–10" | 7–9.5" | 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 275/30R20. Fit 255/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,201 on the road, so it under-counts by 201. At a true 70 mph the first turns 888 times a minute, the second 871.
What drivers reported
Nobody has filed a complaint naming 275/30R20. 255/35R20 carries 4.