315/65R18 vs 325/65R18
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/65R18 | 325/65R18 | Change |
|---|---|---|---|
| Overall height | 34.1" | 34.6" | +1.5 % |
| Sidewall | 8.06" | 8.32" | +3.2 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 107.2" | 108.8" | +1.5 % |
| Revs per mile | 591 | 582 | -1.5 % |
| Axle height | 17.06" | 17.32" | +6.5 mm |
| Approved rim width | 8.5–11.5" | 9–12" | overlap 9–11.5" |
Both are approved on a rim 9" to 11.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 315/65R18. Fit 325/65R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,150 on the road, so it under-counts by 150. At a true 70 mph the first turns 690 times a minute, the second 679.
What drivers reported
Nobody has filed a complaint naming 315/65R18. 325/65R18 carries 2.