305/40R21 vs 285/45R21
Going from one to the other changes the overall height by +1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/40R21 | 285/45R21 | Change |
|---|---|---|---|
| Overall height | 30.6" | 31.1" | +1.6 % |
| Sidewall | 4.80" | 5.05" | +5.1 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 96.2" | 97.7" | +1.6 % |
| Revs per mile | 659 | 649 | -1.6 % |
| Axle height | 15.30" | 15.55" | +6.3 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/40R21. Fit 285/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,161 on the road, so it under-counts by 161. At a true 70 mph the first turns 769 times a minute, the second 757.