325/60R18 vs 315/60R18
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/60R18 | 315/60R18 | Change |
|---|---|---|---|
| Overall height | 33.4" | 32.9" | -1.4 % |
| Sidewall | 7.68" | 7.44" | -3.1 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 104.8" | 103.3" | -1.4 % |
| Revs per mile | 605 | 613 | +1.4 % |
| Axle height | 16.68" | 16.44" | -6.0 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 325/60R18. Fit 315/60R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,858 on the road, so it over-counts by 142. At a true 70 mph the first turns 705 times a minute, the second 716.