345/40R21 vs 355/40R21
Going from one to the other changes the overall height by +1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/40R21 | 355/40R21 | Change |
|---|---|---|---|
| Overall height | 31.9" | 32.2" | +1.0 % |
| Sidewall | 5.43" | 5.59" | +2.9 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 100.1" | 101.1" | +1.0 % |
| Revs per mile | 633 | 627 | -1.0 % |
| Axle height | 15.93" | 16.09" | +4.0 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 345/40R21. Fit 355/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 90.9 mph | +0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,099 on the road, so it under-counts by 99. At a true 70 mph the first turns 738 times a minute, the second 731.