305/45R21 vs 315/40R21
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/45R21 | 315/40R21 | Change |
|---|---|---|---|
| Overall height | 31.8" | 30.9" | -2.8 % |
| Sidewall | 5.40" | 4.96" | -8.2 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 99.9" | 97.1" | -2.8 % |
| Revs per mile | 634 | 652 | +2.8 % |
| Axle height | 15.90" | 15.46" | -11.3 mm |
| Approved rim width | 8.5–11" | 8.5–11.5" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/45R21. Fit 315/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,722 on the road, so it over-counts by 278. At a true 70 mph the first turns 740 times a minute, the second 761.