325/60R18 vs 345/55R18
Going from one to the other changes the overall height by -1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/60R18 | 345/55R18 | Change |
|---|---|---|---|
| Overall height | 33.4" | 32.9" | -1.2 % |
| Sidewall | 7.68" | 7.47" | -2.7 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 104.8" | 103.5" | -1.2 % |
| Revs per mile | 605 | 612 | +1.2 % |
| Axle height | 16.68" | 16.47" | -5.3 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 325/60R18. Fit 345/55R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.9 mph | -1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,876 on the road, so it over-counts by 124. At a true 70 mph the first turns 705 times a minute, the second 714.