305/45R19 vs 325/40R19
Going from one to the other changes the overall height by -1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/45R19 | 325/40R19 | Change |
|---|---|---|---|
| Overall height | 29.8" | 29.2" | -1.9 % |
| Sidewall | 5.40" | 5.12" | -5.3 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 93.6" | 91.8" | -1.9 % |
| Revs per mile | 677 | 690 | +1.9 % |
| Axle height | 14.90" | 14.62" | -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 305/45R19. Fit 325/40R19 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 | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,808 on the road, so it over-counts by 192. At a true 70 mph the first turns 789 times a minute, the second 805.