335/40R21 vs 315/45R21
Going from one to the other changes the overall height by +1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 335/40R21 | 315/45R21 | Change |
|---|---|---|---|
| Overall height | 31.6" | 32.2" | +1.9 % |
| Sidewall | 5.28" | 5.58" | +5.8 % |
| Section width | 335 mm | 315 mm | -6.0 % |
| Circumference | 99.1" | 101.0" | +1.9 % |
| Revs per mile | 639 | 627 | -1.9 % |
| Axle height | 15.78" | 16.08" | +7.8 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 335/40R21. Fit 315/45R21 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 | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,193 on the road, so it under-counts by 193. At a true 70 mph the first turns 746 times a minute, the second 732.