285/50R21 vs 305/50R21
Going from one to the other changes the overall height by +2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/50R21 | 305/50R21 | Change |
|---|---|---|---|
| Overall height | 32.2" | 33.0" | +2.4 % |
| Sidewall | 5.61" | 6.00" | +7.0 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 101.2" | 103.7" | +2.4 % |
| Revs per mile | 626 | 611 | -2.4 % |
| Axle height | 16.11" | 16.50" | +10.0 mm |
| Approved rim width | 8–10.5" | 8.5–11" | 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 285/50R21. Fit 305/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,244 on the road, so it under-counts by 244. At a true 70 mph the first turns 730 times a minute, the second 713.