295/30R20 vs 315/25R20
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/30R20 | 315/25R20 | Change |
|---|---|---|---|
| Overall height | 27.0" | 26.2" | -2.8 % |
| Sidewall | 3.48" | 3.10" | -11.0 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 84.7" | 82.3" | -2.8 % |
| Revs per mile | 748 | 770 | +2.8 % |
| Axle height | 13.48" | 13.10" | -9.8 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 295/30R20. Fit 315/25R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,715 on the road, so it over-counts by 285. At a true 70 mph the first turns 872 times a minute, the second 898.