355/45R18 vs 345/45R18
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 | 355/45R18 | 345/45R18 | Change |
|---|---|---|---|
| Overall height | 30.6" | 30.2" | -1.2 % |
| Sidewall | 6.29" | 6.11" | -2.8 % |
| Section width | 355 mm | 345 mm | -2.8 % |
| Circumference | 96.1" | 95.0" | -1.2 % |
| Revs per mile | 660 | 667 | +1.2 % |
| Axle height | 15.29" | 15.11" | -4.5 mm |
| Approved rim width | 10–13" | 9.5–12.5" | 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 355/45R18. Fit 345/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,884 on the road, so it over-counts by 116. At a true 70 mph the first turns 769 times a minute, the second 778.