315/25R21 vs 335/25R21
Going from one to the other changes the overall height by +1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/25R21 | 335/25R21 | Change |
|---|---|---|---|
| Overall height | 27.2" | 27.6" | +1.4 % |
| Sidewall | 3.10" | 3.30" | +6.3 % |
| Section width | 315 mm | 335 mm | +6.3 % |
| Circumference | 85.5" | 86.7" | +1.4 % |
| Revs per mile | 741 | 731 | -1.4 % |
| Axle height | 13.60" | 13.80" | +5.0 mm |
| Approved rim width | 8.5–11.5" | 9.5–12.5" | 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 315/25R21. Fit 335/25R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,145 on the road, so it under-counts by 145. At a true 70 mph the first turns 865 times a minute, the second 853.