315/25R21 vs 295/30R21
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 | 315/25R21 | 295/30R21 | Change |
|---|---|---|---|
| Overall height | 27.2" | 28.0" | +2.8 % |
| Sidewall | 3.10" | 3.48" | +12.4 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 85.5" | 87.9" | +2.8 % |
| Revs per mile | 741 | 721 | -2.8 % |
| Axle height | 13.60" | 13.98" | +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 315/25R21. Fit 295/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,282 on the road, so it under-counts by 282. At a true 70 mph the first turns 865 times a minute, the second 841.