345/35R20 vs 325/35R20
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 | 345/35R20 | 325/35R20 | Change |
|---|---|---|---|
| Overall height | 29.5" | 29.0" | -1.9 % |
| Sidewall | 4.75" | 4.48" | -5.8 % |
| Section width | 345 mm | 325 mm | -5.8 % |
| Circumference | 92.7" | 91.0" | -1.9 % |
| Revs per mile | 683 | 696 | +1.9 % |
| Axle height | 14.75" | 14.48" | -7.0 mm |
| Approved rim width | 9.5–12.5" | 9–12" | 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 345/35R20. Fit 325/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.1 mph | -0.9 |
| 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 9,813 on the road, so it over-counts by 187. At a true 70 mph the first turns 797 times a minute, the second 813.