225/50R21 vs 245/45R21
Going from one to the other changes the overall height by -0.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/50R21 | 245/45R21 | Change |
|---|---|---|---|
| Overall height | 29.9" | 29.7" | -0.6 % |
| Sidewall | 4.43" | 4.34" | -2.0 % |
| Section width | 225 mm | 245 mm | +8.9 % |
| Circumference | 93.8" | 93.2" | -0.6 % |
| Revs per mile | 675 | 679 | +0.6 % |
| Axle height | 14.93" | 14.84" | -2.3 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 225/50R21. Fit 245/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.7 mph | -0.3 |
| 70 mph | 69.6 mph | -0.4 |
| 90 mph | 89.5 mph | -0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,941 on the road, so it over-counts by 59. At a true 70 mph the first turns 788 times a minute, the second 793.