295/70R20 vs 315/65R20
Going from one to the other changes the overall height by -0.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/70R20 | 315/65R20 | Change |
|---|---|---|---|
| Overall height | 36.3" | 36.1" | -0.4 % |
| Sidewall | 8.13" | 8.06" | -0.8 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 113.9" | 113.5" | -0.4 % |
| Revs per mile | 556 | 558 | +0.4 % |
| Axle height | 18.13" | 18.06" | -1.8 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 295/70R20. Fit 315/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.8 mph | -0.2 |
| 70 mph | 69.7 mph | -0.3 |
| 90 mph | 89.7 mph | -0.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,962 on the road, so it over-counts by 38. At a true 70 mph the first turns 649 times a minute, the second 651.