345/45R18 vs 355/45R18
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 | 345/45R18 | 355/45R18 | Change |
|---|---|---|---|
| Overall height | 30.2" | 30.6" | +1.2 % |
| Sidewall | 6.11" | 6.29" | +2.9 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 95.0" | 96.1" | +1.2 % |
| Revs per mile | 667 | 660 | -1.2 % |
| Axle height | 15.11" | 15.29" | +4.5 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 345/45R18. Fit 355/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,117 on the road, so it under-counts by 117. At a true 70 mph the first turns 778 times a minute, the second 769.