305/30R19 vs 285/30R19
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/30R19 | 285/30R19 | Change |
|---|---|---|---|
| Overall height | 26.2" | 25.7" | -1.8 % |
| Sidewall | 3.60" | 3.37" | -6.6 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 82.3" | 80.8" | -1.8 % |
| Revs per mile | 770 | 784 | +1.8 % |
| Axle height | 13.10" | 12.87" | -6.0 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 305/30R19. Fit 285/30R19 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,820 on the road, so it over-counts by 180. At a true 70 mph the first turns 898 times a minute, the second 914.