225/50R15 vs 205/55R15
Going from one to the other changes the overall height by +0.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/50R15 | 205/55R15 | Change |
|---|---|---|---|
| Overall height | 23.9" | 23.9" | +0.1 % |
| Sidewall | 4.43" | 4.44" | +0.2 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 75.0" | 75.0" | +0.1 % |
| Revs per mile | 845 | 845 | -0.1 % |
| Axle height | 11.93" | 11.94" | +0.3 mm |
| Approved rim width | 6–8.5" | 5.5–7.5" | overlap 6–7.5" |
Both are approved on a rim 6" to 7.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 225/50R15. Fit 205/55R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | +0.0 |
| 50 mph | 50.0 mph | +0.0 |
| 70 mph | 70.1 mph | +0.1 |
| 90 mph | 90.1 mph | +0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,008 on the road, so it under-counts by 8. At a true 70 mph the first turns 986 times a minute, the second 985.