335/30R21 vs 315/35R21
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/30R21 | 315/35R21 | Change |
|---|---|---|---|
| Overall height | 28.9" | 29.7" | +2.7 % |
| Sidewall | 3.96" | 4.34" | +9.7 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 90.8" | 93.2" | +2.7 % |
| Revs per mile | 698 | 679 | -2.7 % |
| Axle height | 14.46" | 14.84" | +9.8 mm |
| Approved rim width | 9.5–12.5" | 8.5–11.5" | overlap 9.5–11.5" |
Both are approved on a rim 9.5" to 11.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 335/30R21. Fit 315/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,266 on the road, so it under-counts by 266. At a true 70 mph the first turns 814 times a minute, the second 793.