315/40R23 vs 305/40R23
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 | 315/40R23 | 305/40R23 | Change |
|---|---|---|---|
| Overall height | 32.9" | 32.6" | -1.0 % |
| Sidewall | 4.96" | 4.80" | -3.2 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 103.4" | 102.4" | -1.0 % |
| Revs per mile | 613 | 619 | +1.0 % |
| Axle height | 16.46" | 16.30" | -4.0 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 315/40R23. Fit 305/40R23 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 9,904 on the road, so it over-counts by 96. At a true 70 mph the first turns 715 times a minute, the second 722.