305/35R20 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 | 305/35R20 | 325/35R20 | Change |
|---|---|---|---|
| Overall height | 28.4" | 29.0" | +1.9 % |
| Sidewall | 4.20" | 4.48" | +6.6 % |
| Section width | 305 mm | 325 mm | +6.6 % |
| Circumference | 89.2" | 91.0" | +1.9 % |
| Revs per mile | 710 | 696 | -1.9 % |
| Axle height | 14.20" | 14.48" | +7.0 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 305/35R20. Fit 325/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,194 on the road, so it under-counts by 194. At a true 70 mph the first turns 828 times a minute, the second 813.