335/50R20 vs 315/55R20
Going from one to the other changes the overall height by +1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/50R20 | 315/55R20 | Change |
|---|---|---|---|
| Overall height | 33.2" | 33.6" | +1.4 % |
| Sidewall | 6.59" | 6.82" | +3.4 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 104.3" | 105.7" | +1.4 % |
| Revs per mile | 608 | 599 | -1.4 % |
| Axle height | 16.59" | 16.82" | +5.8 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 335/50R20. Fit 315/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,136 on the road, so it under-counts by 136. At a true 70 mph the first turns 709 times a minute, the second 699.