325/50R20 vs 315/55R20
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 | 325/50R20 | 315/55R20 | Change |
|---|---|---|---|
| Overall height | 32.8" | 33.6" | +2.6 % |
| Sidewall | 6.40" | 6.82" | +6.6 % |
| Section width | 325 mm | 315 mm | -3.1 % |
| Circumference | 103.0" | 105.7" | +2.6 % |
| Revs per mile | 615 | 599 | -2.6 % |
| Axle height | 16.40" | 16.82" | +10.8 mm |
| Approved rim width | 9–12" | 8.5–11.5" | overlap 9–11.5" |
Both are approved on a rim 9" 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 325/50R20. Fit 315/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,258 on the road, so it under-counts by 258. At a true 70 mph the first turns 717 times a minute, the second 699.
What drivers reported
325/50R20 carries 1 complaints. Nobody has filed one naming 315/55R20.