245/60R20 vs 255/55R20
Going from one to the other changes the overall height by -1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/60R20 | 255/55R20 | Change |
|---|---|---|---|
| Overall height | 31.6" | 31.0" | -1.7 % |
| Sidewall | 5.79" | 5.52" | -4.6 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 99.2" | 97.5" | -1.7 % |
| Revs per mile | 639 | 650 | +1.7 % |
| Axle height | 15.79" | 15.52" | -6.8 mm |
| Approved rim width | 7–9" | 7–9.5" | 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 245/60R20. Fit 255/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.5 mph | -1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,832 on the road, so it over-counts by 168. At a true 70 mph the first turns 745 times a minute, the second 758.
What drivers reported
Nobody has filed a complaint naming 245/60R20. 255/55R20 carries 2.