335/40R18 vs 325/40R18
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 | 335/40R18 | 325/40R18 | Change |
|---|---|---|---|
| Overall height | 28.6" | 28.2" | -1.1 % |
| Sidewall | 5.28" | 5.12" | -3.0 % |
| Section width | 335 mm | 325 mm | -3.0 % |
| Circumference | 89.7" | 88.7" | -1.1 % |
| Revs per mile | 706 | 714 | +1.1 % |
| Axle height | 14.28" | 14.12" | -4.0 mm |
| Approved rim width | 9.5–12.5" | 9–12" | overlap 9.5–12" |
Both are approved on a rim 9.5" to 12" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 335/40R18. Fit 325/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,890 on the road, so it over-counts by 110. At a true 70 mph the first turns 824 times a minute, the second 833.