335/35R18 vs 315/40R18
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/35R18 | 315/40R18 | Change |
|---|---|---|---|
| Overall height | 27.2" | 27.9" | +2.5 % |
| Sidewall | 4.62" | 4.96" | +7.5 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 85.6" | 87.7" | +2.5 % |
| Revs per mile | 741 | 722 | -2.5 % |
| Axle height | 13.62" | 13.96" | +8.8 mm |
| Approved rim width | 9.5–12.5" | 8.5–11.5" | overlap 9.5–11.5" |
Both are approved on a rim 9.5" to 11.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 335/35R18. Fit 315/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,253 on the road, so it under-counts by 253. At a true 70 mph the first turns 864 times a minute, the second 843.