325/40R20 vs 345/35R20
Going from one to the other changes the overall height by -2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/40R20 | 345/35R20 | Change |
|---|---|---|---|
| Overall height | 30.2" | 29.5" | -2.4 % |
| Sidewall | 5.12" | 4.75" | -7.1 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 95.0" | 92.7" | -2.4 % |
| Revs per mile | 667 | 683 | +2.4 % |
| Axle height | 15.12" | 14.75" | -9.3 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 325/40R20. Fit 345/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,759 on the road, so it over-counts by 241. At a true 70 mph the first turns 778 times a minute, the second 797.