315/50R21 vs 305/50R21
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 | 315/50R21 | 305/50R21 | Change |
|---|---|---|---|
| Overall height | 33.4" | 33.0" | -1.2 % |
| Sidewall | 6.20" | 6.00" | -3.2 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 104.9" | 103.7" | -1.2 % |
| Revs per mile | 604 | 611 | +1.2 % |
| Axle height | 16.70" | 16.50" | -5.0 mm |
| Approved rim width | 8.5–11.5" | 8.5–11" | 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 315/50R21. Fit 305/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 88.9 mph | -1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,882 on the road, so it over-counts by 118. At a true 70 mph the first turns 704 times a minute, the second 713.