355/40R21 vs 345/40R21
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 | 355/40R21 | 345/40R21 | Change |
|---|---|---|---|
| Overall height | 32.2" | 31.9" | -1.0 % |
| Sidewall | 5.59" | 5.43" | -2.8 % |
| Section width | 355 mm | 345 mm | -2.8 % |
| Circumference | 101.1" | 100.1" | -1.0 % |
| Revs per mile | 627 | 633 | +1.0 % |
| Axle height | 16.09" | 15.93" | -4.0 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 355/40R21. Fit 345/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,902 on the road, so it over-counts by 98. At a true 70 mph the first turns 731 times a minute, the second 738.