315/30R21 vs 305/30R21
Going from one to the other changes the overall height by -0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/30R21 | 305/30R21 | Change |
|---|---|---|---|
| Overall height | 28.4" | 28.2" | -0.8 % |
| Sidewall | 3.72" | 3.60" | -3.2 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 89.3" | 88.6" | -0.8 % |
| Revs per mile | 709 | 715 | +0.8 % |
| Axle height | 14.22" | 14.10" | -3.0 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 315/30R21. Fit 305/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.3 mph | -0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,917 on the road, so it over-counts by 83. At a true 70 mph the first turns 827 times a minute, the second 834.