315/35R23 vs 305/35R23
Going from one to the other changes the overall height by -0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/35R23 | 305/35R23 | Change |
|---|---|---|---|
| Overall height | 31.7" | 31.4" | -0.9 % |
| Sidewall | 4.34" | 4.20" | -3.2 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 99.5" | 98.7" | -0.9 % |
| Revs per mile | 637 | 642 | +0.9 % |
| Axle height | 15.84" | 15.70" | -3.5 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 315/35R23. Fit 305/35R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,913 on the road, so it over-counts by 87. At a true 70 mph the first turns 743 times a minute, the second 749.