285/30R18 vs 305/25R18
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/30R18 | 305/25R18 | Change |
|---|---|---|---|
| Overall height | 24.7" | 24.0" | -2.9 % |
| Sidewall | 3.37" | 3.00" | -10.8 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 77.7" | 75.4" | -2.9 % |
| Revs per mile | 815 | 840 | +2.9 % |
| Axle height | 12.37" | 12.00" | -9.3 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 285/30R18. Fit 305/25R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,706 on the road, so it over-counts by 294. At a true 70 mph the first turns 951 times a minute, the second 980.