335/60R20 vs 315/60R20
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/60R20 | 315/60R20 | Change |
|---|---|---|---|
| Overall height | 35.8" | 34.9" | -2.6 % |
| Sidewall | 7.91" | 7.44" | -6.0 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 112.6" | 109.6" | -2.6 % |
| Revs per mile | 563 | 578 | +2.6 % |
| Axle height | 17.91" | 17.44" | -12.0 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 335/60R20. Fit 315/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,736 on the road, so it over-counts by 264. At a true 70 mph the first turns 657 times a minute, the second 675.