345/50R21 vs 355/45R21
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/50R21 | 355/45R21 | Change |
|---|---|---|---|
| Overall height | 34.6" | 33.6" | -2.9 % |
| Sidewall | 6.79" | 6.29" | -7.4 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 108.6" | 105.5" | -2.9 % |
| Revs per mile | 583 | 601 | +2.9 % |
| Axle height | 17.29" | 16.79" | -12.8 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 345/50R21. Fit 355/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,710 on the road, so it over-counts by 290. At a true 70 mph the first turns 680 times a minute, the second 701.