355/30R20 vs 345/30R20
Going from one to the other changes the overall height by -0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 355/30R20 | 345/30R20 | Change |
|---|---|---|---|
| Overall height | 28.4" | 28.1" | -0.8 % |
| Sidewall | 4.19" | 4.07" | -2.8 % |
| Section width | 355 mm | 345 mm | -2.8 % |
| Circumference | 89.2" | 88.4" | -0.8 % |
| Revs per mile | 710 | 716 | +0.8 % |
| Axle height | 14.19" | 14.07" | -3.0 mm |
| Approved rim width | 10–13" | 9.5–12.5" | overlap 10–12.5" |
Both are approved on a rim 10" to 12.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 355/30R20. Fit 345/30R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.3 mph | -0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,917 on the road, so it over-counts by 83. At a true 70 mph the first turns 829 times a minute, the second 836.