305/65R21 vs 315/60R21
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/65R21 | 315/60R21 | Change |
|---|---|---|---|
| Overall height | 36.6" | 35.9" | -2.0 % |
| Sidewall | 7.81" | 7.44" | -4.7 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 115.0" | 112.7" | -2.0 % |
| Revs per mile | 551 | 562 | +2.0 % |
| Axle height | 18.31" | 17.94" | -9.3 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 315/60R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,801 on the road, so it over-counts by 199. At a true 70 mph the first turns 643 times a minute, the second 656.