315/45R23 vs 305/45R23
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/45R23 | 305/45R23 | Change |
|---|---|---|---|
| Overall height | 34.2" | 33.8" | -1.0 % |
| Sidewall | 5.58" | 5.40" | -3.2 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 107.3" | 106.2" | -1.0 % |
| Revs per mile | 590 | 597 | +1.0 % |
| Axle height | 17.08" | 16.90" | -4.5 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 315/45R23. Fit 305/45R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,896 on the road, so it over-counts by 104. At a true 70 mph the first turns 689 times a minute, the second 696.