335/25R21 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 | 335/25R21 | 345/25R21 | Change |
|---|---|---|---|
| Overall height | 27.6" | 27.8" | +0.7 % |
| Sidewall | 3.30" | 3.40" | +3.0 % |
| Section width | 335 mm | 345 mm | +3.0 % |
| Circumference | 86.7" | 87.3" | +0.7 % |
| Revs per mile | 731 | 726 | -0.7 % |
| Axle height | 13.80" | 13.90" | +2.5 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 335/25R21. Fit 345/25R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.6 mph | +0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,071 on the road, so it under-counts by 71. At a true 70 mph the first turns 853 times a minute, the second 847.