355/25R21 vs 345/25R21
Going from one to the other changes the overall height by -0.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 355/25R21 | 345/25R21 | Change |
|---|---|---|---|
| Overall height | 28.0" | 27.8" | -0.7 % |
| Sidewall | 3.49" | 3.40" | -2.8 % |
| Section width | 355 mm | 345 mm | -2.8 % |
| Circumference | 87.9" | 87.3" | -0.7 % |
| Revs per mile | 721 | 726 | +0.7 % |
| Axle height | 13.99" | 13.90" | -2.5 mm |
| Approved rim width | 10–13" | 9.5–12.5" | 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 355/25R21. Fit 345/25R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.5 mph | -0.5 |
| 90 mph | 89.4 mph | -0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,930 on the road, so it over-counts by 70. At a true 70 mph the first turns 841 times a minute, the second 847.