325/25R21 vs 345/25R21
Going from one to the other changes the overall height by +1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 325/25R21 | 345/25R21 | Change |
|---|---|---|---|
| Overall height | 27.4" | 27.8" | +1.4 % |
| Sidewall | 3.20" | 3.40" | +6.2 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 86.1" | 87.3" | +1.4 % |
| Revs per mile | 736 | 726 | -1.4 % |
| Axle height | 13.70" | 13.90" | +5.0 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 325/25R21. Fit 345/25R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,144 on the road, so it under-counts by 144. At a true 70 mph the first turns 859 times a minute, the second 847.