235/50R15 vs 225/55R15
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 | 235/50R15 | 225/55R15 | Change |
|---|---|---|---|
| Overall height | 24.3" | 24.7" | +2.0 % |
| Sidewall | 4.63" | 4.87" | +5.3 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 76.2" | 77.7" | +2.0 % |
| Revs per mile | 832 | 815 | -2.0 % |
| Axle height | 12.13" | 12.37" | +6.3 mm |
| Approved rim width | 6.5–8.5" | 6–8.5" | 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 235/50R15. Fit 225/55R15 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 10,203 on the road, so it under-counts by 203. At a true 70 mph the first turns 970 times a minute, the second 951.