255/45R15 vs 245/50R15
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/45R15 | 245/50R15 | Change |
|---|---|---|---|
| Overall height | 24.0" | 24.6" | +2.5 % |
| Sidewall | 4.52" | 4.82" | +6.8 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 75.5" | 77.4" | +2.5 % |
| Revs per mile | 839 | 818 | -2.5 % |
| Axle height | 12.02" | 12.32" | +7.8 mm |
| Approved rim width | 7–9.5" | 7–9" | 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 255/45R15. Fit 245/50R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,254 on the road, so it under-counts by 254. At a true 70 mph the first turns 979 times a minute, the second 955.