295/40R21 vs 315/40R21
Going from one to the other changes the overall height by +2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/40R21 | 315/40R21 | Change |
|---|---|---|---|
| Overall height | 30.3" | 30.9" | +2.1 % |
| Sidewall | 4.65" | 4.96" | +6.8 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 95.2" | 97.1" | +2.1 % |
| Revs per mile | 666 | 652 | -2.1 % |
| Axle height | 15.15" | 15.46" | +8.0 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 295/40R21. Fit 315/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 91.9 mph | +1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,208 on the road, so it under-counts by 208. At a true 70 mph the first turns 777 times a minute, the second 761.
What drivers reported
295/40R21 carries 2 complaints. Nobody has filed one naming 315/40R21.