335/50R18 vs 325/55R18
Going from one to the other changes the overall height by +2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/50R18 | 325/55R18 | Change |
|---|---|---|---|
| Overall height | 31.2" | 32.1" | +2.8 % |
| Sidewall | 6.59" | 7.04" | +6.7 % |
| Section width | 335 mm | 325 mm | -3.0 % |
| Circumference | 98.0" | 100.8" | +2.8 % |
| Revs per mile | 647 | 629 | -2.8 % |
| Axle height | 15.59" | 16.04" | +11.3 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 335/50R18. Fit 325/55R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,284 on the road, so it under-counts by 284. At a true 70 mph the first turns 754 times a minute, the second 734.