335/25R18 vs 315/25R18
Going from one to the other changes the overall height by -1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/25R18 | 315/25R18 | Change |
|---|---|---|---|
| Overall height | 24.6" | 24.2" | -1.6 % |
| Sidewall | 3.30" | 3.10" | -6.0 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 77.3" | 76.0" | -1.6 % |
| Revs per mile | 820 | 833 | +1.6 % |
| Axle height | 12.30" | 12.10" | -5.0 mm |
| Approved rim width | 9.5–12.5" | 8.5–11.5" | overlap 9.5–11.5" |
Both are approved on a rim 9.5" 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 335/25R18. Fit 315/25R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,840 on the road, so it over-counts by 160. At a true 70 mph the first turns 957 times a minute, the second 972.