325/65R18 vs 305/65R18
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/65R18 | 305/65R18 | Change |
|---|---|---|---|
| Overall height | 34.6" | 33.6" | -3.0 % |
| Sidewall | 8.32" | 7.81" | -6.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 108.8" | 105.6" | -3.0 % |
| Revs per mile | 582 | 600 | +3.0 % |
| Axle height | 17.32" | 16.81" | -13.0 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 325/65R18. Fit 305/65R18 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 9,704 on the road, so it over-counts by 296. At a true 70 mph the first turns 679 times a minute, the second 700.
What drivers reported
325/65R18 carries 2 complaints. Nobody has filed one naming 305/65R18.