255/30R20 vs 245/35R20
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 | 255/30R20 | 245/35R20 | Change |
|---|---|---|---|
| Overall height | 26.0" | 26.8" | +2.8 % |
| Sidewall | 3.01" | 3.38" | +12.1 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 81.8" | 84.0" | +2.8 % |
| Revs per mile | 775 | 754 | -2.8 % |
| Axle height | 13.01" | 13.38" | +9.3 mm |
| Approved rim width | 7–9.5" | 7–9" | overlap 7–9" |
Both are approved on a rim 7" to 9" 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 245/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,280 on the road, so it under-counts by 280. At a true 70 mph the first turns 904 times a minute, the second 880.
What drivers reported
Nobody has filed a complaint naming 255/30R20. 245/35R20 carries 6.