275/50R18 vs 285/45R18
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 | 275/50R18 | 285/45R18 | Change |
|---|---|---|---|
| Overall height | 28.8" | 28.1" | -2.5 % |
| Sidewall | 5.41" | 5.05" | -6.7 % |
| Section width | 275 mm | 285 mm | +3.6 % |
| Circumference | 90.6" | 88.3" | -2.5 % |
| Revs per mile | 700 | 718 | +2.5 % |
| Axle height | 14.41" | 14.05" | -9.3 mm |
| Approved rim width | 7.5–10" | 8–10.5" | overlap 8–10" |
Both are approved on a rim 8" to 10" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 275/50R18. Fit 285/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,747 on the road, so it over-counts by 253. At a true 70 mph the first turns 816 times a minute, the second 837.