335/55R21 vs 315/55R21
Going from one to the other changes the overall height by -2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/55R21 | 315/55R21 | Change |
|---|---|---|---|
| Overall height | 35.5" | 34.6" | -2.4 % |
| Sidewall | 7.25" | 6.82" | -6.0 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 111.6" | 108.8" | -2.4 % |
| Revs per mile | 568 | 582 | +2.4 % |
| Axle height | 17.75" | 17.32" | -11.0 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 335/55R21. Fit 315/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,756 on the road, so it over-counts by 244. At a true 70 mph the first turns 663 times a minute, the second 679.