325/45R20 vs 305/50R20
Going from one to the other changes the overall height by +1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/45R20 | 305/50R20 | Change |
|---|---|---|---|
| Overall height | 31.5" | 32.0" | +1.6 % |
| Sidewall | 5.76" | 6.00" | +4.3 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 99.0" | 100.6" | +1.6 % |
| Revs per mile | 640 | 630 | -1.6 % |
| Axle height | 15.76" | 16.00" | +6.3 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 325/45R20. Fit 305/50R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,156 on the road, so it under-counts by 156. At a true 70 mph the first turns 747 times a minute, the second 735.
What drivers reported
Nobody has filed a complaint naming 325/45R20. 305/50R20 carries 4.