335/45R20 vs 315/50R20
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/45R20 | 315/50R20 | Change |
|---|---|---|---|
| Overall height | 31.9" | 32.4" | +1.7 % |
| Sidewall | 5.94" | 6.20" | +4.5 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 100.1" | 101.8" | +1.7 % |
| Revs per mile | 633 | 622 | -1.7 % |
| Axle height | 15.94" | 16.20" | +6.8 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 335/45R20. Fit 315/50R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,167 on the road, so it under-counts by 167. At a true 70 mph the first turns 738 times a minute, the second 726.