335/50R21 vs 345/45R21
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/50R21 | 345/45R21 | Change |
|---|---|---|---|
| Overall height | 34.2" | 33.2" | -2.8 % |
| Sidewall | 6.59" | 6.11" | -7.3 % |
| Section width | 335 mm | 345 mm | +3.0 % |
| Circumference | 107.4" | 104.4" | -2.8 % |
| Revs per mile | 590 | 607 | +2.8 % |
| Axle height | 17.09" | 16.61" | -12.3 mm |
| Approved rim width | 9.5–12.5" | 9.5–12.5" | overlap 9.5–12.5" |
Both are approved on a rim 9.5" 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 335/50R21. Fit 345/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,718 on the road, so it over-counts by 282. At a true 70 mph the first turns 688 times a minute, the second 708.