295/60R21 vs 315/55R21
Going from one to the other changes the overall height by -0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/60R21 | 315/55R21 | Change |
|---|---|---|---|
| Overall height | 34.9" | 34.6" | -0.8 % |
| Sidewall | 6.97" | 6.82" | -2.1 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 109.8" | 108.8" | -0.8 % |
| Revs per mile | 577 | 582 | +0.8 % |
| Axle height | 17.47" | 17.32" | -3.8 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 295/60R21. Fit 315/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,915 on the road, so it over-counts by 85. At a true 70 mph the first turns 673 times a minute, the second 679.