295/55R20 vs 315/50R20
Going from one to the other changes the overall height by -1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/55R20 | 315/50R20 | Change |
|---|---|---|---|
| Overall height | 32.8" | 32.4" | -1.1 % |
| Sidewall | 6.39" | 6.20" | -2.9 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 103.0" | 101.8" | -1.1 % |
| Revs per mile | 615 | 622 | +1.1 % |
| Axle height | 16.39" | 16.20" | -4.8 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 295/55R20. Fit 315/50R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,886 on the road, so it over-counts by 114. At a true 70 mph the first turns 718 times a minute, the second 726.