295/50R21 vs 315/45R21
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/50R21 | 315/45R21 | Change |
|---|---|---|---|
| Overall height | 32.6" | 32.2" | -1.4 % |
| Sidewall | 5.81" | 5.58" | -3.9 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 102.5" | 101.0" | -1.4 % |
| Revs per mile | 618 | 627 | +1.4 % |
| Axle height | 16.31" | 16.08" | -5.8 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 295/50R21. Fit 315/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,861 on the road, so it over-counts by 139. At a true 70 mph the first turns 721 times a minute, the second 732.