305/55R20 vs 315/55R20
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/55R20 | 315/55R20 | Change |
|---|---|---|---|
| Overall height | 33.2" | 33.6" | +1.3 % |
| Sidewall | 6.60" | 6.82" | +3.3 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 104.3" | 105.7" | +1.3 % |
| Revs per mile | 607 | 599 | -1.3 % |
| Axle height | 16.60" | 16.82" | +5.5 mm |
| Approved rim width | 8.5–11" | 8.5–11.5" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/55R20. 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 | 70.9 mph | +0.9 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,130 on the road, so it under-counts by 130. At a true 70 mph the first turns 709 times a minute, the second 699.
What drivers reported
305/55R20 carries 1 complaints. Nobody has filed one naming 315/55R20.