305/45R18 vs 325/40R18
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/45R18 | 325/40R18 | Change |
|---|---|---|---|
| Overall height | 28.8" | 28.2" | -2.0 % |
| Sidewall | 5.40" | 5.12" | -5.3 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 90.5" | 88.7" | -2.0 % |
| Revs per mile | 700 | 714 | +2.0 % |
| Axle height | 14.40" | 14.12" | -7.3 mm |
| Approved rim width | 8.5–11" | 9–12" | overlap 9–11" |
Both are approved on a rim 9" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/45R18. Fit 325/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,802 on the road, so it over-counts by 198. At a true 70 mph the first turns 817 times a minute, the second 833.