305/35R18 vs 325/30R18
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 | 305/35R18 | 325/30R18 | Change |
|---|---|---|---|
| Overall height | 26.4" | 25.7" | -2.8 % |
| Sidewall | 4.20" | 3.84" | -8.7 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 83.0" | 80.7" | -2.8 % |
| Revs per mile | 764 | 785 | +2.8 % |
| Axle height | 13.20" | 12.84" | -9.3 mm |
| Approved rim width | 8.5–11" | 9–12" | 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 305/35R18. Fit 325/30R18 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 | 68.1 mph | -1.9 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,724 on the road, so it over-counts by 276. At a true 70 mph the first turns 891 times a minute, the second 916.
What drivers reported
305/35R18 carries 2 complaints. Nobody has filed one naming 325/30R18.