235/60R20 vs 235/55R20
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/60R20 | 235/55R20 | Change |
|---|---|---|---|
| Overall height | 31.1" | 30.2" | -3.0 % |
| Sidewall | 5.55" | 5.09" | -8.3 % |
| Section width | 235 mm | 235 mm | 0.0 % |
| Circumference | 97.7" | 94.8" | -3.0 % |
| Revs per mile | 648 | 668 | +3.0 % |
| Axle height | 15.55" | 15.09" | -11.8 mm |
| Approved rim width | 6.5–8.5" | 6.5–8.5" | overlap 6.5–8.5" |
Both are approved on a rim 6.5" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/60R20. Fit 235/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,703 on the road, so it over-counts by 297. At a true 70 mph the first turns 757 times a minute, the second 780.