305/40R17 vs 315/40R17
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/40R17 | 315/40R17 | Change |
|---|---|---|---|
| Overall height | 26.6" | 26.9" | +1.2 % |
| Sidewall | 4.80" | 4.96" | +3.3 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 83.6" | 84.6" | +1.2 % |
| Revs per mile | 758 | 749 | -1.2 % |
| Axle height | 13.30" | 13.46" | +4.0 mm |
| Approved rim width | 8.5–11" | 8.5–11.5" | 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 305/40R17. Fit 315/40R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,118 on the road, so it under-counts by 118. At a true 70 mph the first turns 884 times a minute, the second 874.