295/60R21 vs 315/60R21
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 | 295/60R21 | 315/60R21 | Change |
|---|---|---|---|
| Overall height | 34.9" | 35.9" | +2.7 % |
| Sidewall | 6.97" | 7.44" | +6.8 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 109.8" | 112.7" | +2.7 % |
| Revs per mile | 577 | 562 | -2.7 % |
| Axle height | 17.47" | 17.94" | +12.0 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 295/60R21. Fit 315/60R21 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 | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,270 on the road, so it under-counts by 270. At a true 70 mph the first turns 673 times a minute, the second 656.