345/55R18 vs 355/55R18
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/55R18 | 355/55R18 | Change |
|---|---|---|---|
| Overall height | 32.9" | 33.4" | +1.3 % |
| Sidewall | 7.47" | 7.69" | +2.9 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 103.5" | 104.8" | +1.3 % |
| Revs per mile | 612 | 604 | -1.3 % |
| Axle height | 16.47" | 16.69" | +5.5 mm |
| Approved rim width | 9.5–12.5" | 10–13" | overlap 10–12.5" |
Both are approved on a rim 10" to 12.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 345/55R18. Fit 355/55R18 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 | 70.9 mph | +0.9 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,131 on the road, so it under-counts by 131. At a true 70 mph the first turns 714 times a minute, the second 705.