325/50R21 vs 345/45R21
Going from one to the other changes the overall height by -1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/50R21 | 345/45R21 | Change |
|---|---|---|---|
| Overall height | 33.8" | 33.2" | -1.7 % |
| Sidewall | 6.40" | 6.11" | -4.5 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 106.2" | 104.4" | -1.7 % |
| Revs per mile | 597 | 607 | +1.7 % |
| Axle height | 16.90" | 16.61" | -7.3 mm |
| Approved rim width | 9–12" | 9.5–12.5" | overlap 9.5–12" |
Both are approved on a rim 9.5" to 12" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 325/50R21. Fit 345/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.5 mph | -1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,831 on the road, so it over-counts by 169. At a true 70 mph the first turns 696 times a minute, the second 708.