305/65R21 vs 315/65R21
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 | 305/65R21 | 315/65R21 | Change |
|---|---|---|---|
| Overall height | 36.6" | 37.1" | +1.4 % |
| Sidewall | 7.81" | 8.06" | +3.3 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 115.0" | 116.6" | +1.4 % |
| Revs per mile | 551 | 543 | -1.4 % |
| Axle height | 18.31" | 18.56" | +6.5 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 305/65R21. Fit 315/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,140 on the road, so it under-counts by 140. At a true 70 mph the first turns 643 times a minute, the second 634.