355/55R20 vs 345/60R20
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 | 355/55R20 | 345/60R20 | Change |
|---|---|---|---|
| Overall height | 35.4" | 36.3" | +2.6 % |
| Sidewall | 7.69" | 8.15" | +6.0 % |
| Section width | 355 mm | 345 mm | -2.8 % |
| Circumference | 111.1" | 114.0" | +2.6 % |
| Revs per mile | 570 | 556 | -2.6 % |
| Axle height | 17.69" | 18.15" | +11.8 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 355/55R20. Fit 345/60R20 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 10,262 on the road, so it under-counts by 262. At a true 70 mph the first turns 665 times a minute, the second 648.