335/60R20 vs 315/65R20
Going from one to the other changes the overall height by +0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/60R20 | 315/65R20 | Change |
|---|---|---|---|
| Overall height | 35.8" | 36.1" | +0.8 % |
| Sidewall | 7.91" | 8.06" | +1.9 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 112.6" | 113.5" | +0.8 % |
| Revs per mile | 563 | 558 | -0.8 % |
| Axle height | 17.91" | 18.06" | +3.8 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 335/60R20. Fit 315/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,082 on the road, so it under-counts by 82. At a true 70 mph the first turns 657 times a minute, the second 651.