345/30R18 vs 355/30R18
Going from one to the other changes the overall height by +0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/30R18 | 355/30R18 | Change |
|---|---|---|---|
| Overall height | 26.1" | 26.4" | +0.9 % |
| Sidewall | 4.07" | 4.19" | +2.9 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 82.2" | 82.9" | +0.9 % |
| Revs per mile | 771 | 764 | -0.9 % |
| Axle height | 13.07" | 13.19" | +3.0 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 345/30R18. Fit 355/30R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.8 mph | +0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,090 on the road, so it under-counts by 90. At a true 70 mph the first turns 900 times a minute, the second 892.