315/40R21 vs 305/45R21
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/40R21 | 305/45R21 | Change |
|---|---|---|---|
| Overall height | 30.9" | 31.8" | +2.9 % |
| Sidewall | 4.96" | 5.40" | +8.9 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 97.1" | 99.9" | +2.9 % |
| Revs per mile | 652 | 634 | -2.9 % |
| Axle height | 15.46" | 15.90" | +11.3 mm |
| Approved rim width | 8.5–11.5" | 8.5–11" | 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 315/40R21. Fit 305/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,286 on the road, so it under-counts by 286. At a true 70 mph the first turns 761 times a minute, the second 740.