295/30R21 vs 315/25R21
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/30R21 | 315/25R21 | Change |
|---|---|---|---|
| Overall height | 28.0" | 27.2" | -2.7 % |
| Sidewall | 3.48" | 3.10" | -11.0 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 87.9" | 85.5" | -2.7 % |
| Revs per mile | 721 | 741 | +2.7 % |
| Axle height | 13.98" | 13.60" | -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 295/30R21. Fit 315/25R21 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 9,726 on the road, so it over-counts by 274. At a true 70 mph the first turns 841 times a minute, the second 865.