335/35R20 vs 315/40R20
Going from one to the other changes the overall height by +2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/35R20 | 315/40R20 | Change |
|---|---|---|---|
| Overall height | 29.2" | 29.9" | +2.4 % |
| Sidewall | 4.62" | 4.96" | +7.5 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 91.8" | 94.0" | +2.4 % |
| Revs per mile | 690 | 674 | -2.4 % |
| Axle height | 14.62" | 14.96" | +8.8 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 335/35R20. Fit 315/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,236 on the road, so it under-counts by 236. At a true 70 mph the first turns 805 times a minute, the second 786.