305/45R18 vs 285/50R18
Going from one to the other changes the overall height by +1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/45R18 | 285/50R18 | Change |
|---|---|---|---|
| Overall height | 28.8" | 29.2" | +1.4 % |
| Sidewall | 5.40" | 5.61" | +3.8 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 90.5" | 91.8" | +1.4 % |
| Revs per mile | 700 | 690 | -1.4 % |
| Axle height | 14.40" | 14.61" | +5.3 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/45R18. Fit 285/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,144 on the road, so it under-counts by 144. At a true 70 mph the first turns 817 times a minute, the second 805.