315/30R20 vs 295/30R20
Going from one to the other changes the overall height by -1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/30R20 | 295/30R20 | Change |
|---|---|---|---|
| Overall height | 27.4" | 27.0" | -1.7 % |
| Sidewall | 3.72" | 3.48" | -6.3 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 86.2" | 84.7" | -1.7 % |
| Revs per mile | 735 | 748 | +1.7 % |
| Axle height | 13.72" | 13.48" | -6.0 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 315/30R20. Fit 295/30R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.5 mph | -1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,828 on the road, so it over-counts by 172. At a true 70 mph the first turns 857 times a minute, the second 872.