255/45R18 vs 245/50R18
Going from one to the other changes the overall height by +2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/45R18 | 245/50R18 | Change |
|---|---|---|---|
| Overall height | 27.0" | 27.6" | +2.3 % |
| Sidewall | 4.52" | 4.82" | +6.8 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 84.9" | 86.9" | +2.3 % |
| Revs per mile | 746 | 730 | -2.3 % |
| Axle height | 13.52" | 13.82" | +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 255/45R18. Fit 245/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.0 mph | +2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,226 on the road, so it under-counts by 226. At a true 70 mph the first turns 870 times a minute, the second 851.
What drivers reported
255/45R18 carries 6 complaints. Nobody has filed one naming 245/50R18.