325/40R18 vs 305/45R18
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 | 325/40R18 | 305/45R18 | Change |
|---|---|---|---|
| Overall height | 28.2" | 28.8" | +2.0 % |
| Sidewall | 5.12" | 5.40" | +5.6 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 88.7" | 90.5" | +2.0 % |
| Revs per mile | 714 | 700 | -2.0 % |
| Axle height | 14.12" | 14.40" | +7.3 mm |
| Approved rim width | 9–12" | 8.5–11" | 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 325/40R18. Fit 305/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,202 on the road, so it under-counts by 202. At a true 70 mph the first turns 833 times a minute, the second 817.