315/65R20 vs 305/65R20
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/65R20 | 305/65R20 | Change |
|---|---|---|---|
| Overall height | 36.1" | 35.6" | -1.4 % |
| Sidewall | 8.06" | 7.81" | -3.2 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 113.5" | 111.9" | -1.4 % |
| Revs per mile | 558 | 566 | +1.4 % |
| Axle height | 18.06" | 17.81" | -6.5 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 315/65R20. Fit 305/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,858 on the road, so it over-counts by 142. At a true 70 mph the first turns 651 times a minute, the second 661.