325/50R18 vs 345/45R18
Going from one to the other changes the overall height by -1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/50R18 | 345/45R18 | Change |
|---|---|---|---|
| Overall height | 30.8" | 30.2" | -1.9 % |
| Sidewall | 6.40" | 6.11" | -4.5 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 96.7" | 95.0" | -1.9 % |
| Revs per mile | 655 | 667 | +1.9 % |
| Axle height | 15.40" | 15.11" | -7.3 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 325/50R18. Fit 345/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,815 on the road, so it over-counts by 185. At a true 70 mph the first turns 764 times a minute, the second 778.