325/25R18 vs 345/25R18
Going from one to the other changes the overall height by +1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/25R18 | 345/25R18 | Change |
|---|---|---|---|
| Overall height | 24.4" | 24.8" | +1.6 % |
| Sidewall | 3.20" | 3.40" | +6.2 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 76.6" | 77.9" | +1.6 % |
| Revs per mile | 827 | 814 | -1.6 % |
| Axle height | 12.20" | 12.40" | +5.0 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 325/25R18. Fit 345/25R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,161 on the road, so it under-counts by 161. At a true 70 mph the first turns 964 times a minute, the second 949.