305/30R21 vs 315/30R21
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 | 305/30R21 | 315/30R21 | Change |
|---|---|---|---|
| Overall height | 28.2" | 28.4" | +0.8 % |
| Sidewall | 3.60" | 3.72" | +3.3 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 88.6" | 89.3" | +0.8 % |
| Revs per mile | 715 | 709 | -0.8 % |
| Axle height | 14.10" | 14.22" | +3.0 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 305/30R21. Fit 315/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.8 mph | +0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,084 on the road, so it under-counts by 84. At a true 70 mph the first turns 834 times a minute, the second 827.