325/40R18 vs 305/40R18
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/40R18 | 305/40R18 | Change |
|---|---|---|---|
| Overall height | 28.2" | 27.6" | -2.2 % |
| Sidewall | 5.12" | 4.80" | -6.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 88.7" | 86.7" | -2.2 % |
| Revs per mile | 714 | 731 | +2.2 % |
| Axle height | 14.12" | 13.80" | -8.0 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 305/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,777 on the road, so it over-counts by 223. At a true 70 mph the first turns 833 times a minute, the second 852.