225/55R21 vs 245/50R21
Going from one to the other changes the overall height by -0.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/55R21 | 245/50R21 | Change |
|---|---|---|---|
| Overall height | 30.7" | 30.6" | -0.3 % |
| Sidewall | 4.87" | 4.82" | -1.0 % |
| Section width | 225 mm | 245 mm | +8.9 % |
| Circumference | 96.6" | 96.3" | -0.3 % |
| Revs per mile | 656 | 658 | +0.3 % |
| Axle height | 15.37" | 15.32" | -1.3 mm |
| Approved rim width | 6–8.5" | 7–9" | 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 225/55R21. Fit 245/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.8 mph | -0.2 |
| 70 mph | 69.8 mph | -0.2 |
| 90 mph | 89.7 mph | -0.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,968 on the road, so it over-counts by 32. At a true 70 mph the first turns 765 times a minute, the second 768.