315/65R21 vs 295/65R21
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 | 315/65R21 | 295/65R21 | Change |
|---|---|---|---|
| Overall height | 37.1" | 36.1" | -2.8 % |
| Sidewall | 8.06" | 7.55" | -6.3 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 116.6" | 113.4" | -2.8 % |
| Revs per mile | 543 | 559 | +2.8 % |
| Axle height | 18.56" | 18.05" | -13.0 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 315/65R21. Fit 295/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,724 on the road, so it over-counts by 276. At a true 70 mph the first turns 634 times a minute, the second 652.