325/45R18 vs 345/40R18
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/45R18 | 345/40R18 | Change |
|---|---|---|---|
| Overall height | 29.5" | 28.9" | -2.2 % |
| Sidewall | 5.76" | 5.43" | -5.6 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 92.7" | 90.7" | -2.2 % |
| Revs per mile | 683 | 699 | +2.2 % |
| Axle height | 14.76" | 14.43" | -8.3 mm |
| Approved rim width | 9–12" | 9.5–12.5" | 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 325/45R18. Fit 345/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,780 on the road, so it over-counts by 220. At a true 70 mph the first turns 797 times a minute, the second 815.