245/50R20 vs 225/55R20
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 | 245/50R20 | 225/55R20 | Change |
|---|---|---|---|
| Overall height | 29.6" | 29.7" | +0.3 % |
| Sidewall | 4.82" | 4.87" | +1.0 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 93.1" | 93.4" | +0.3 % |
| Revs per mile | 680 | 678 | -0.3 % |
| Axle height | 14.82" | 14.87" | +1.3 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 245/50R20. Fit 225/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.2 mph | +0.2 |
| 70 mph | 70.2 mph | +0.2 |
| 90 mph | 90.3 mph | +0.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,033 on the road, so it under-counts by 33. At a true 70 mph the first turns 794 times a minute, the second 791.