335/40R21 vs 355/40R21
Going from one to the other changes the overall height by +2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/40R21 | 355/40R21 | Change |
|---|---|---|---|
| Overall height | 31.6" | 32.2" | +2.0 % |
| Sidewall | 5.28" | 5.59" | +6.0 % |
| Section width | 335 mm | 355 mm | +6.0 % |
| Circumference | 99.1" | 101.1" | +2.0 % |
| Revs per mile | 639 | 627 | -2.0 % |
| Axle height | 15.78" | 16.09" | +8.0 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 335/40R21. Fit 355/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,200 on the road, so it under-counts by 200. At a true 70 mph the first turns 746 times a minute, the second 731.