345/30R21 vs 355/30R21
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 | 345/30R21 | 355/30R21 | Change |
|---|---|---|---|
| Overall height | 29.1" | 29.4" | +0.8 % |
| Sidewall | 4.07" | 4.19" | +2.9 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 91.6" | 92.3" | +0.8 % |
| Revs per mile | 692 | 686 | -0.8 % |
| Axle height | 14.57" | 14.69" | +3.0 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 345/30R21. Fit 355/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,081 on the road, so it under-counts by 81. At a true 70 mph the first turns 807 times a minute, the second 801.