345/45R21 vs 355/45R21
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/45R21 | 355/45R21 | Change |
|---|---|---|---|
| Overall height | 33.2" | 33.6" | +1.1 % |
| Sidewall | 6.11" | 6.29" | +2.9 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 104.4" | 105.5" | +1.1 % |
| Revs per mile | 607 | 601 | -1.1 % |
| Axle height | 16.61" | 16.79" | +4.5 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 345/45R21. Fit 355/45R21 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 | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,107 on the road, so it under-counts by 107. At a true 70 mph the first turns 708 times a minute, the second 701.