285/55R18 vs 305/50R18
Going from one to the other changes the overall height by -1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/55R18 | 305/50R18 | Change |
|---|---|---|---|
| Overall height | 30.3" | 30.0" | -1.1 % |
| Sidewall | 6.17" | 6.00" | -2.7 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 95.3" | 94.3" | -1.1 % |
| Revs per mile | 665 | 672 | +1.1 % |
| Axle height | 15.17" | 15.00" | -4.3 mm |
| Approved rim width | 8–10.5" | 8.5–11" | 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 285/55R18. Fit 305/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,890 on the road, so it over-counts by 110. At a true 70 mph the first turns 775 times a minute, the second 784.