245/50R21 vs 255/45R21
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/50R21 | 255/45R21 | Change |
|---|---|---|---|
| Overall height | 30.6" | 30.0" | -2.0 % |
| Sidewall | 4.82" | 4.52" | -6.3 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 96.3" | 94.4" | -2.0 % |
| Revs per mile | 658 | 671 | +2.0 % |
| Axle height | 15.32" | 15.02" | -7.8 mm |
| Approved rim width | 7–9" | 7–9.5" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/50R21. Fit 255/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,801 on the road, so it over-counts by 199. At a true 70 mph the first turns 768 times a minute, the second 783.