335/40R18 vs 345/40R18
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/40R18 | 345/40R18 | Change |
|---|---|---|---|
| Overall height | 28.6" | 28.9" | +1.1 % |
| Sidewall | 5.28" | 5.43" | +3.0 % |
| Section width | 335 mm | 345 mm | +3.0 % |
| Circumference | 89.7" | 90.7" | +1.1 % |
| Revs per mile | 706 | 699 | -1.1 % |
| Axle height | 14.28" | 14.43" | +4.0 mm |
| Approved rim width | 9.5–12.5" | 9.5–12.5" | overlap 9.5–12.5" |
Both are approved on a rim 9.5" 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 335/40R18. Fit 345/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,110 on the road, so it under-counts by 110. At a true 70 mph the first turns 824 times a minute, the second 815.