325/50R18 vs 315/50R18
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/50R18 | 315/50R18 | Change |
|---|---|---|---|
| Overall height | 30.8" | 30.4" | -1.3 % |
| Sidewall | 6.40" | 6.20" | -3.1 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 96.7" | 95.5" | -1.3 % |
| Revs per mile | 655 | 663 | +1.3 % |
| Axle height | 15.40" | 15.20" | -5.0 mm |
| Approved rim width | 9–12" | 8.5–11.5" | 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 325/50R18. Fit 315/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,872 on the road, so it over-counts by 128. At a true 70 mph the first turns 764 times a minute, the second 774.