305/40R18 vs 315/40R18
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/40R18 | 315/40R18 | Change |
|---|---|---|---|
| Overall height | 27.6" | 27.9" | +1.1 % |
| Sidewall | 4.80" | 4.96" | +3.3 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 86.7" | 87.7" | +1.1 % |
| Revs per mile | 731 | 722 | -1.1 % |
| Axle height | 13.80" | 13.96" | +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 305/40R18. Fit 315/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,114 on the road, so it under-counts by 114. At a true 70 mph the first turns 852 times a minute, the second 843.