315/45R21 vs 305/50R21
Going from one to the other changes the overall height by +2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/45R21 | 305/50R21 | Change |
|---|---|---|---|
| Overall height | 32.2" | 33.0" | +2.6 % |
| Sidewall | 5.58" | 6.00" | +7.6 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 101.0" | 103.7" | +2.6 % |
| Revs per mile | 627 | 611 | -2.6 % |
| Axle height | 16.08" | 16.50" | +10.8 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 315/45R21. Fit 305/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,263 on the road, so it under-counts by 263. At a true 70 mph the first turns 732 times a minute, the second 713.