305/50R21 vs 315/50R21
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/50R21 | 315/50R21 | Change |
|---|---|---|---|
| Overall height | 33.0" | 33.4" | +1.2 % |
| Sidewall | 6.00" | 6.20" | +3.3 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 103.7" | 104.9" | +1.2 % |
| Revs per mile | 611 | 604 | -1.2 % |
| Axle height | 16.50" | 16.70" | +5.0 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 305/50R21. Fit 315/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,119 on the road, so it under-counts by 119. At a true 70 mph the first turns 713 times a minute, the second 704.