315/60R21 vs 305/65R21
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 | 315/60R21 | 305/65R21 | Change |
|---|---|---|---|
| Overall height | 35.9" | 36.6" | +2.0 % |
| Sidewall | 7.44" | 7.81" | +4.9 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 112.7" | 115.0" | +2.0 % |
| Revs per mile | 562 | 551 | -2.0 % |
| Axle height | 17.94" | 18.31" | +9.3 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 315/60R21. Fit 305/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,203 on the road, so it under-counts by 203. At a true 70 mph the first turns 656 times a minute, the second 643.