325/65R18 vs 345/65R18
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 | 345/65R18 | Change |
|---|---|---|---|
| Overall height | 34.6" | 35.7" | +3.0 % |
| Sidewall | 8.32" | 8.83" | +6.2 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 108.8" | 112.0" | +3.0 % |
| Revs per mile | 582 | 566 | -3.0 % |
| Axle height | 17.32" | 17.83" | +13.0 mm |
| Approved rim width | 9–12" | 9.5–12.5" | overlap 9.5–12" |
Both are approved on a rim 9.5" to 12" 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 345/65R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.7 mph | +2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,296 on the road, so it under-counts by 296. At a true 70 mph the first turns 679 times a minute, the second 660.
What drivers reported
325/65R18 carries 2 complaints. Nobody has filed one naming 345/65R18.