325/25R20 vs 345/25R20
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/25R20 | 345/25R20 | Change |
|---|---|---|---|
| Overall height | 26.4" | 26.8" | +1.5 % |
| Sidewall | 3.20" | 3.40" | +6.2 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 82.9" | 84.2" | +1.5 % |
| Revs per mile | 764 | 753 | -1.5 % |
| Axle height | 13.20" | 13.40" | +5.0 mm |
| Approved rim width | 9–12" | 9.5–12.5" | overlap 9.5–12" |
Both are approved on a rim 9.5" to 12" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 325/25R20. Fit 345/25R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,149 on the road, so it under-counts by 149. At a true 70 mph the first turns 891 times a minute, the second 878.