225/60R21 vs 225/55R21
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/60R21 | 225/55R21 | Change |
|---|---|---|---|
| Overall height | 31.6" | 30.7" | -2.8 % |
| Sidewall | 5.31" | 4.87" | -8.3 % |
| Section width | 225 mm | 225 mm | 0.0 % |
| Circumference | 99.4" | 96.6" | -2.8 % |
| Revs per mile | 638 | 656 | +2.8 % |
| Axle height | 15.81" | 15.37" | -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 225/60R21. Fit 225/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,720 on the road, so it over-counts by 280. At a true 70 mph the first turns 744 times a minute, the second 765.