345/65R21 vs 355/60R21
Going from one to the other changes the overall height by -2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 345/65R21 | 355/60R21 | Change |
|---|---|---|---|
| Overall height | 38.7" | 37.8" | -2.3 % |
| Sidewall | 8.83" | 8.39" | -5.0 % |
| Section width | 345 mm | 355 mm | +2.9 % |
| Circumference | 121.4" | 118.7" | -2.3 % |
| Revs per mile | 522 | 534 | +2.3 % |
| Axle height | 19.33" | 18.89" | -11.3 mm |
| Approved rim width | 9.5–12.5" | 10–13" | 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 345/65R21. Fit 355/60R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 87.9 mph | -2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,771 on the road, so it over-counts by 229. At a true 70 mph the first turns 609 times a minute, the second 623.