355/35R21 vs 335/40R21
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 355/35R21 | 335/40R21 | Change |
|---|---|---|---|
| Overall height | 30.8" | 31.6" | +2.5 % |
| Sidewall | 4.89" | 5.28" | +7.8 % |
| Section width | 355 mm | 335 mm | -5.6 % |
| Circumference | 96.7" | 99.1" | +2.5 % |
| Revs per mile | 655 | 639 | -2.5 % |
| Axle height | 15.39" | 15.78" | +9.8 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 355/35R21. Fit 335/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,249 on the road, so it under-counts by 249. At a true 70 mph the first turns 764 times a minute, the second 746.