345/60R20 vs 355/60R20
Going from one to the other changes the overall height by +1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/60R20 | 355/60R20 | Change |
|---|---|---|---|
| Overall height | 36.3" | 36.8" | +1.3 % |
| Sidewall | 8.15" | 8.39" | +2.9 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 114.0" | 115.5" | +1.3 % |
| Revs per mile | 556 | 548 | -1.3 % |
| Axle height | 18.15" | 18.39" | +6.0 mm |
| Approved rim width | 9.5–12.5" | 10–13" | 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 345/60R20. Fit 355/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 70.9 mph | +0.9 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,130 on the road, so it under-counts by 130. At a true 70 mph the first turns 648 times a minute, the second 640.