225/55R21 vs 225/50R21
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/55R21 | 225/50R21 | Change |
|---|---|---|---|
| Overall height | 30.7" | 29.9" | -2.9 % |
| Sidewall | 4.87" | 4.43" | -9.1 % |
| Section width | 225 mm | 225 mm | 0.0 % |
| Circumference | 96.6" | 93.8" | -2.9 % |
| Revs per mile | 656 | 675 | +2.9 % |
| Axle height | 15.37" | 14.93" | -11.3 mm |
| Approved rim width | 6–8.5" | 6–8.5" | overlap 6–8.5" |
Both are approved on a rim 6" 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 225/55R21. Fit 225/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,712 on the road, so it over-counts by 288. At a true 70 mph the first turns 765 times a minute, the second 788.