325/40R20 vs 305/45R20
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 | 325/40R20 | 305/45R20 | Change |
|---|---|---|---|
| Overall height | 30.2" | 30.8" | +1.9 % |
| Sidewall | 5.12" | 5.40" | +5.6 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 95.0" | 96.8" | +1.9 % |
| Revs per mile | 667 | 655 | -1.9 % |
| Axle height | 15.12" | 15.40" | +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 325/40R20. Fit 305/45R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,189 on the road, so it under-counts by 189. At a true 70 mph the first turns 778 times a minute, the second 764.