305/40R18 vs 325/40R18
Going from one to the other changes the overall height by +2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/40R18 | 325/40R18 | Change |
|---|---|---|---|
| Overall height | 27.6" | 28.2" | +2.3 % |
| Sidewall | 4.80" | 5.12" | +6.6 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 86.7" | 88.7" | +2.3 % |
| Revs per mile | 731 | 714 | -2.3 % |
| Axle height | 13.80" | 14.12" | +8.0 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 305/40R18. Fit 325/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,228 on the road, so it under-counts by 228. At a true 70 mph the first turns 852 times a minute, the second 833.