255/45R20 vs 245/50R20
Going from one to the other changes the overall height by +2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/45R20 | 245/50R20 | Change |
|---|---|---|---|
| Overall height | 29.0" | 29.6" | +2.1 % |
| Sidewall | 4.52" | 4.82" | +6.8 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 91.2" | 93.1" | +2.1 % |
| Revs per mile | 695 | 680 | -2.1 % |
| Axle height | 14.52" | 14.82" | +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 255/45R20. Fit 245/50R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 91.9 mph | +1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,210 on the road, so it under-counts by 210. At a true 70 mph the first turns 810 times a minute, the second 794.