345/30R18 vs 335/30R18
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 | 335/30R18 | Change |
|---|---|---|---|
| Overall height | 26.1" | 25.9" | -0.9 % |
| Sidewall | 4.07" | 3.96" | -2.9 % |
| Section width | 345 mm | 335 mm | -2.9 % |
| Circumference | 82.2" | 81.4" | -0.9 % |
| Revs per mile | 771 | 778 | +0.9 % |
| Axle height | 13.07" | 12.96" | -3.0 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 345/30R18. Fit 335/30R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,910 on the road, so it over-counts by 90. At a true 70 mph the first turns 900 times a minute, the second 908.