345/55R21 vs 355/55R21
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/55R21 | 355/55R21 | Change |
|---|---|---|---|
| Overall height | 35.9" | 36.4" | +1.2 % |
| Sidewall | 7.47" | 7.69" | +2.9 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 112.9" | 114.3" | +1.2 % |
| Revs per mile | 561 | 554 | -1.2 % |
| Axle height | 17.97" | 18.19" | +5.5 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 345/55R21. Fit 355/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,120 on the road, so it under-counts by 120. At a true 70 mph the first turns 655 times a minute, the second 647.