315/50R18 vs 325/45R18
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/50R18 | 325/45R18 | Change |
|---|---|---|---|
| Overall height | 30.4" | 29.5" | -2.9 % |
| Sidewall | 6.20" | 5.76" | -7.1 % |
| Section width | 315 mm | 325 mm | +3.2 % |
| Circumference | 95.5" | 92.7" | -2.9 % |
| Revs per mile | 663 | 683 | +2.9 % |
| Axle height | 15.20" | 14.76" | -11.3 mm |
| Approved rim width | 8.5–11.5" | 9–12" | overlap 9–11.5" |
Both are approved on a rim 9" to 11.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 315/50R18. Fit 325/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,709 on the road, so it over-counts by 291. At a true 70 mph the first turns 774 times a minute, the second 797.