255/50R20 vs 235/55R20
Going from one to the other changes the overall height by +0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/50R20 | 235/55R20 | Change |
|---|---|---|---|
| Overall height | 30.0" | 30.2" | +0.5 % |
| Sidewall | 5.02" | 5.09" | +1.4 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 94.4" | 94.8" | +0.5 % |
| Revs per mile | 671 | 668 | -0.5 % |
| Axle height | 15.02" | 15.09" | +1.8 mm |
| Approved rim width | 7–9.5" | 6.5–8.5" | overlap 7–8.5" |
Both are approved on a rim 7" 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 255/50R20. Fit 235/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.2 mph | +0.2 |
| 70 mph | 70.3 mph | +0.3 |
| 90 mph | 90.4 mph | +0.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,046 on the road, so it under-counts by 46. At a true 70 mph the first turns 783 times a minute, the second 780.