325/60R18 vs 305/60R18
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/60R18 | 305/60R18 | Change |
|---|---|---|---|
| Overall height | 33.4" | 32.4" | -2.8 % |
| Sidewall | 7.68" | 7.20" | -6.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 104.8" | 101.8" | -2.8 % |
| Revs per mile | 605 | 622 | +2.8 % |
| Axle height | 16.68" | 16.20" | -12.0 mm |
| Approved rim width | 9–12" | 8.5–11" | overlap 9–11" |
Both are approved on a rim 9" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 325/60R18. Fit 305/60R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,717 on the road, so it over-counts by 283. At a true 70 mph the first turns 705 times a minute, the second 726.