315/65R23 vs 305/65R23
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/65R23 | 305/65R23 | Change |
|---|---|---|---|
| Overall height | 39.1" | 38.6" | -1.3 % |
| Sidewall | 8.06" | 7.81" | -3.2 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 122.9" | 121.3" | -1.3 % |
| Revs per mile | 516 | 522 | +1.3 % |
| Axle height | 19.56" | 19.31" | -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 315/65R23. Fit 305/65R23 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 | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,869 on the road, so it over-counts by 131. At a true 70 mph the first turns 601 times a minute, the second 609.