335/65R20 vs 315/65R20
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/65R20 | 315/65R20 | Change |
|---|---|---|---|
| Overall height | 37.1" | 36.1" | -2.8 % |
| Sidewall | 8.57" | 8.06" | -6.0 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 116.7" | 113.5" | -2.8 % |
| Revs per mile | 543 | 558 | +2.8 % |
| Axle height | 18.57" | 18.06" | -13.0 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 335/65R20. Fit 315/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,724 on the road, so it over-counts by 276. At a true 70 mph the first turns 633 times a minute, the second 651.