245/55R21 vs 225/60R21
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 | 245/55R21 | 225/60R21 | Change |
|---|---|---|---|
| Overall height | 31.6" | 31.6" | +0.1 % |
| Sidewall | 5.31" | 5.31" | +0.2 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 99.3" | 99.4" | +0.1 % |
| Revs per mile | 638 | 638 | -0.1 % |
| Axle height | 15.81" | 15.81" | +0.3 mm |
| Approved rim width | 7–9" | 6–8.5" | 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 245/55R21. Fit 225/60R21 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 | 70.0 mph | +0.0 |
| 90 mph | 90.1 mph | +0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,006 on the road, so it under-counts by 6. At a true 70 mph the first turns 744 times a minute, the second 744.