315/25R20 vs 295/30R20
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/25R20 | 295/30R20 | Change |
|---|---|---|---|
| Overall height | 26.2" | 27.0" | +2.9 % |
| Sidewall | 3.10" | 3.48" | +12.4 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 82.3" | 84.7" | +2.9 % |
| Revs per mile | 770 | 748 | -2.9 % |
| Axle height | 13.10" | 13.48" | +9.8 mm |
| Approved rim width | 8.5–11.5" | 8.5–11" | 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 315/25R20. Fit 295/30R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,293 on the road, so it under-counts by 293. At a true 70 mph the first turns 898 times a minute, the second 872.