325/30R20 vs 305/35R20
Going from one to the other changes the overall height by +2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/30R20 | 305/35R20 | Change |
|---|---|---|---|
| Overall height | 27.7" | 28.4" | +2.6 % |
| Sidewall | 3.84" | 4.20" | +9.5 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 87.0" | 89.2" | +2.6 % |
| Revs per mile | 729 | 710 | -2.6 % |
| Axle height | 13.84" | 14.20" | +9.3 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 325/30R20. Fit 305/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,263 on the road, so it under-counts by 263. At a true 70 mph the first turns 850 times a minute, the second 828.