305/65R20 vs 315/60R20
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 | 305/65R20 | 315/60R20 | Change |
|---|---|---|---|
| Overall height | 35.6" | 34.9" | -2.0 % |
| Sidewall | 7.81" | 7.44" | -4.7 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 111.9" | 109.6" | -2.0 % |
| Revs per mile | 566 | 578 | +2.0 % |
| Axle height | 17.81" | 17.44" | -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 305/65R20. Fit 315/60R20 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 9,795 on the road, so it over-counts by 205. At a true 70 mph the first turns 661 times a minute, the second 675.