345/35R18 vs 325/40R18
Going from one to the other changes the overall height by +2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/35R18 | 325/40R18 | Change |
|---|---|---|---|
| Overall height | 27.5" | 28.2" | +2.6 % |
| Sidewall | 4.75" | 5.12" | +7.7 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 86.4" | 88.7" | +2.6 % |
| Revs per mile | 733 | 714 | -2.6 % |
| Axle height | 13.75" | 14.12" | +9.3 mm |
| Approved rim width | 9.5–12.5" | 9–12" | overlap 9.5–12" |
Both are approved on a rim 9.5" to 12" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 345/35R18. Fit 325/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,265 on the road, so it under-counts by 265. At a true 70 mph the first turns 855 times a minute, the second 833.