325/35R18 vs 315/35R18
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/35R18 | 315/35R18 | Change |
|---|---|---|---|
| Overall height | 27.0" | 26.7" | -1.0 % |
| Sidewall | 4.48" | 4.34" | -3.1 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 84.7" | 83.8" | -1.0 % |
| Revs per mile | 748 | 756 | +1.0 % |
| Axle height | 13.48" | 13.34" | -3.5 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 325/35R18. Fit 315/35R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,898 on the road, so it over-counts by 102. At a true 70 mph the first turns 873 times a minute, the second 882.